Optimized Heteroepitaxial growth of semiconductors

ABSTRACT

A method of performing heteroepitaxy comprises exposing a substrate to a carrier gas, a first precursor gas, a Group II/III element, and a second precursor gas, to form a heteroepitaxial growth of one of GaAs, AlAs, InAs, GaP, InP, ZnSe, GaSe, CdSe, InSe, ZnTe, CdTe, GaTe, HgTe, GaSb, InSb, AlSb, CdS, GaN, and AlN on the substrate; wherein the substrate comprises one of GaAs, AlAs, InAs, GaP, InP, ZnSe, GaSe, CdSe, InSe, ZnTe, CdTe, GaTe, HgTe, GaSb, InSb, AlSb, CdS, GaN, and AlN; wherein the carrier gas is H2, wherein the first precursor is HCl, the Group II/III element comprises at least one of Zn, Cd, Hg, Al, Ga, and In; and wherein the second precursor is one of AsH3 (arsine), PH3 (phosphine), H2Se (hydrogen selenide), H2Te (hydrogen telluride), SbH3 (hydrogen antimonide), H2S (hydrogen sulfide), and NH3 (ammonia). The process may be an HVPE (hydride vapor phase epitaxy) process.

PRIORITY

Pursuant to 37 C.F.R. § 1.78(a)(4), this application claims the benefitof and priority to prior filed Provisional Application Ser. No.62/681,155, filed 6 Jun. 2018, and co-pending Non-Provisionalapplication Ser. No. 16/201,446, filed 27 Nov. 2018, each of which areexpressly incorporated herein by reference.

RIGHTS OF THE GOVERNMENT

The invention described herein may be manufactured and used by or forthe Government of the United States for all governmental purposeswithout the payment of any royalty.

FIELD OF THE INVENTION

The present invention relates generally to heteroepitaxial growth ofsemiconductor materials and, more particularly, to a process for thickheteroepitaxial growth, e.g. hundreds of microns thick, of semiconductormaterials based on in-situ pre-growth treatment of the substrate.

BACKGROUND OF THE INVENTION

Heteroepitaxy, i.e. the growth of one material on another material, hashad a remarkable impact on optics and electronics. It is the firstchoice and sometimes the only available option when there is lack of anative substrate, e.g. in the cases of gallium nitride (GaN) andaluminum nitride (AlN), which are typically grown on sapphire (Al₂O₃) oron silicon carbide (SiC), and in other material cases as well. However,in some cases, even when native substrates are readily available,heteroepitaxy might still be preferable. One convincing example is thenumerous attempts to grow different semiconductor materials on silicon(Si).

As a chemical element, Si is widely distributed in nature; silicon isthe cheapest and the most common substrate, and has the highest possiblematerial quality. Silicon can be grown conveniently from melt in theshape of large boules (up to 450 mm in diameter) or plates in matureindustrial processes such as CZ (Czochralski) and EFG (edge-definedfilm-fed growth). However, what really makes Si an attractive substratefor heteroepitaxy is its high electrical and thermal conductivity andthe possibility to combine it with some optoelectronic materials, e.g.gallium arsenide (GaAs) or gallium phosphide (GaP) and their ternariesand quaternaries. Accordingly, the growth of a number of electronic andoptoelectronic materials (including GaAs and GaP) have been attempted onsilicon substrates using different growth techniques. Among these growthtechniques are well-known industrial techniques such as Molecular BeamEpitaxy (MBE) and Metal Organic Chemical Vapor Deposition (MOCVD), aswell as some other less typical approaches, e.g. Remote Plasma-EnhancedChemical Vapor Deposition (RPECVD) or Liquid Phase Epitaxy (LPE). Theopposite cases of growth of silicon on other suitable materials,including GaAs and GaP, have also been investigated.

The most frequent attempts of growth on silicon substrates wereperformed either directly on the Si-substrate or after the deposition ofan intermediate buffer layer (described immediately below), in order toaccommodate the growing layer to the foreign substrate (Si), so a greatdeal of effort has been made in engineering those buffer layers. In somecases when the lattice mismatches between the Si-substrate and thegrowing layer were really large, for example, in the case of GaAs growthon Si (4.2% lattice mismatch) growth was performed on so-calledpatterned templates. (Note: Herein, a “substrate” is defined as a plain,ready-for-epitaxial-growth surface, while a “template” is a substratethat has a periodic or aperiodic structure called a “pattern” depositedor otherwise formed on its surface). The pattern on the template issupposed to provide more uniform nucleation on the template surface andmore efficient release of the initial strain that results from a largelattice and thermal mismatch between substrate and the growing layer.This technique is reported to work well with other materials even atreally large lattice mismatches, such as in the case of the growth ofGaSb on patterned GaAs templates (˜7% lattice mismatch).

The choice of an alternative substrate material, when possible, avoidsthe fast oxidation of Si and the subsequent effort to remove the oxidelayer. However, with or without surface oxidation, a lot can still bedone in each specific case to prepare the substrate surface in a waythat will facilitate the initial nucleation that determines thesubsequent stages of growth. Prior growth treatment of the foreignsubstrate and prior growth of a low-temperature (LT) buffer layer, forexample, is applied during the growth of GaN on sapphire; it was foundhelpful to pretreat the sapphire surface with an amount of ammonia(NH₃). This process, called “nitridation”, aims to convert the sapphire(Al₂O₃) surface into AlN (or aluminum nitride), which has a latticeconstant that is much closer to the lattice constant of GaN thansapphire. The nitridation process is followed by the deposition of theaforementioned low-temperature (LT) AlN or GaN buffer layer which servesto release the strain built during the initial stage of heteroepitaxy.The next step is the deposition of the intended thick high-temperature(HT) GaN layer.

Next to the lattice mismatch, the thermal mismatch (this is thedifference in the thermal expansion coefficients of the substrate andlayer materials) is another factor that along with the difference in thethermal conductivity should be taken into account when attemptingheteroepitaxial thick-growth because it can result in layer cracking.Due to all these factors, neglecting to some extent the HVPE techniques,research has been focused over the years on using either thin growthtechniques such as MOCVD or MBE, or other alternative solutions forachieving thick structures such as, for example, the wafer bondingtechnique or in some rear cases the PVD technique. All these techniques,of course, have their own disadvantages. Respectively, growths of someother materials, such as ZnSe, GaAs, GaP, GaSb and their ternaries oneach other, mostly by MOCVD and MBE, have also been reported. In theparticular case of nonlinear optical (NLO) materials such as GaAs, GaP,or ZnSe, when the pursued applications were related to frequencyconversion devices, the growth should be performed onorientation-patterned (OP) templates. The pattern on such templatesconsists of parallel-striped areas (domains) withperiodically-alternated crystal polarity. In such cases it is importantthat the thick HVPE growth is capable of delivering the patternthroughout the whole layer thickness while maintaining good domainfidelity and gaining a large enough aperture for the pump beam that will“ignite” the frequency conversion (FC) processes to propagate. So inthis particular case these patterns play a specific optical role andhave nothing to do with the patterned templates mentioned above, whichaim to facilitate the initial stages of growth in cases when the relatedmismatches are too large.

In the case of growths on OP-templates, the heteroepitaxial approach ispreferred for at least two reasons already mentioned: 1) lack of nativeOP-templates or 2) other advantages of combining two different materialin one quasi-phase matching (QPM) structure—such advantages could be thebetter quality and/or the lower market price of the wafers, theavailability of closely-matching non-native OP templates, a more maturegrowth process, etc. Due to the first reason, PVD layer growth (PVD ismore known as a bulk growth technique) of OPZnSe has been performed onOPGaAs templates with a great deal of confidence, because of the smalllattice mismatch (+0.24%) between ZnSe and GaAs. More timid attemptswere made to grow thick GaP on plain GaAs substrates and OPGaP on OPGaAstemplates without much hope for success due to the larger latticemismatch between GaAs and GaP (−3.57%). Surprisingly, while the firstwork yielded relatively poor domain fidelity and, as a consequence,limited optical results, the results from the second attempt were ratherencouraging, especially when the growth was performed on plain GaAssubstrates.

Unfortunately, at this time the resulting layers were not thick enoughfor a practical frequency conversion demonstration. Thus OPGaP/OPGaAsheteroepitaxy was neglected for several more years, and researchreturned to homoepitaxy of OPGaP/OPGaP. However, the preparation ofOPGaP templates for the OPGaP/OPGaP homoepitaxy revealed somesignificant shortcomings. Such shortcomings include the low quality(poor parallelism and high etch pit density) and high price of availableGaP wafers, the presence of an additional absorption band in the IRbetween 2 and 4 μm, the absence of an etch-stop material (needed tosecure the thickness of the inverted layer during the preparation ofOP-templates by the wafer-bonded techniques), etc. As a consequence,using such wafers for the fabrication of OP-templates unavoidablyresulted in the same poor OPGaP template quality and, subsequently, inpoor HVPE growths on them. Thus the idea to use the 8-10 times cheaperbut much higher-quality OPGaAs templates, which had been fabricatedroutinely for a number of years for OPGaAs/OPGaAs homoepitaxy, came backto the stage again—this time in support of the OPGaP/OPGaAsheteroepitaxy. As a consequence, after making some suitable changes inthe reactor configuration and the growth chemistry excellent domainfidelity were repeatedly grown on OPGaAs templates.

Accordingly, the previous work leaves much to be desired with regard toheteroepitaxial thick growth opportunities. At this point, developmentin many areas of optics and electronics appeared to be almost to theirlimits, but a lot of unexplored potential remains for HVPE heteroepitaxythrough further engineering of the buffer layer or through otherless-explored approaches, especially in the cases of larger lattice andthermal mismatches between the substrate and the growing layer. Someexamples: (i) in optoelectronics: proper combinations of electronic withoptoelectronic materials realized by heteroepitaxy may take over otherless efficient approaches, e.g. bonding or PVD. This can result infurther miniaturization of ultrafast all-optically communicatingprocessors and other devices; (ii) in the solar cell industry: eventhough Si can be grown easily in mature processes, e.g. CZ and EFG, andis still considered as an indisputable leader among the rest of the“solar” materials, there are many material limitations that restrict theefficiency of the Si-solar cells. In this case, some optimizedheteroepitaxial approaches for the growth of other materials on Si maylead to small-dimension, high-power, broad-band hybrid solar cellssuitable for various unachieved applications; (iii) multi-materialheterostructures can also favor development of multi-color detectorsthat could cover a wide range of the spectrum; (iv) in the field ofdevelopment of new laser sources: heteroepitaxy may allow growths ofphase matching materials (including of such that have never been grownpractically in large monocrystalline substrates) or quasi-phase matchingstructures and the design of high power, broadly tunable frequencyconversion devices. Such devices could easily achieve new frequencyranges resulting in various new applications in areas such as defense,security, industry, science, and medicine.

The mechanisms of crystal growth are complex. On a practical level, itis difficult to determine on an atomic scale what exactly occurs afteronly the first few monolayers of growth, even in the simplest cases ofhomoepitaxy of plain semiconductor materials, e.g. Si and Ge. Obviously,heteroepitaxy is a more complex event. This is the real reason why thesemiconductor industry has adopted only a few techniques for slow, thingrowth, e.g. MOCVD and MBE, and for only a few well-studied materials.This is also the reason why thick growth approaches, e.g. HVPE, are usedin only a limited number of homo- and heteroepitaxial cases. In reality,HVPE has never been accepted as a “full” member of the industrialfamily. This is because HVPE and the other thick growth techniques alsohave their own problems. An example of such problems is that in mostcases, thick HVPE growth requires a two-step process—first to deposit onthe substrate a thin MOCVD or MBE buffer layer or, to first deposit a LTlow-quality HVPE buffer layer.

SUMMARY OF THE INVENTION

The present invention overcomes the foregoing problems and othershortcomings, drawbacks, and challenges of heteroepitaxial growth ofsemiconductor materials. While the invention will be described inconnection with certain embodiments, it will be understood that theinvention is not limited to these embodiments. To the contrary, thisinvention includes all alternatives, modifications, and equivalents asmay be included within the spirit and scope of the present invention.

The inventive one-step thick HVPE growth process is preceded by anin-situ pre-growth treatment of the substrate, disclosed and explainedbelow, and is supported by results that confirm that this approachensures a smooth transition between the growing layer and the foreignsubstrates in many material cases, while avoiding other steps withunfavorable impacts on the quality of the growing layer. The disclosedprocess demonstrates that in many cases heteroepitaxy could be thebetter choice nevertheless that in some cases homoepitaxy is possible.Thus, based on the results presented below it is suggested that thecommon belief that homoepitaxy is always preferable over heteroepitaxyshould be reconsidered.

Due to the complex chemistry and specific growth issues for eachmaterial, HVPE is not a traditional industrial technique such as MOCVDand MBE. However, it is the only known approach (excluding wafer bondingor to a small extend the Physical Vapor Deposition (PVD) technique)capable of providing layers hundreds of microns thick needed for theapplication discussed below. However, due the nature of its fast growthand, as a result, poor control of the initial stage of growth, HVPEshould often be combined with either growth on patterned templates, ordeposition first of a low or high temperature intermediate buffer layer,which could be with gradually changing composition. As aclose-to-equilibrium process, HVPE could be also combined with afar-from-equilibrium technique such as MOCVD or MBE. In contrast toHVPE, such techniques by providing high supersaturation conditions canbe used to deposit on the substrate first an initial thin layer from thesame or even other (better matching) material in many cases when HVPEcannot do that. At this point one should bear in mind that there aresome significant differences between the MBE (or MOCVD) and the HVPEbuffer layers—the first ones are much thinner (30-370 nm) than thesecond ones which could achieve 3-5 μm thickness or more. In addition,while the MBE (or MOCVD) layers are in most of the cases from the samematerial and are used mostly for reducing the initial mismatch strain,the HVPE layers can be from the same or different than the parenting(the growing and the substrate) materials and can be used moreefficiently rather to fit the substrate to the growing material bygradually changing their composition over the layer thickness. In thispoint of view the definition for “changing the composition” of thebuffer layer may have two different meanings. For example, if we grow alow temperature (LT) GaN buffer layer to prepare the sapphire substratefor the real high temperature (HT) GaN growth, under “changing thecomposition” we have in mind changing the V-III ratio during the bufferlayer growth. The situation is completely different during heteroepitaxywhen we try to grow, for example GaP on a GaAs substrate. Than we changethe composition of the buffer layer with increasing the phosphoruscontent on the expense of the arsine content, so in this case the bufferlayer is a GaAs_(x)P_(1-x) ternary with gradually changing composition.

According to one embodiment of the present invention a method ofperforming heteroepitaxy, comprises exposing a substrate exposing asubstrate to a carrier gas, a first precursor gas, a Group II/IIIelement, and a second precursor gas, to form a heteroepitaxial growth ofone of GaAs, AlAs, InAs, GaP, InP, ZnSe, GaSe, CdSe, InSe, ZnTe, CdTe,GaTe, HgTe, GaSb, InSb, AlSb, CdS, GaN, and AlN on the substrate;wherein the substrate comprises one of GaAs, AlAs, InAs, GaP, InP, ZnSe,GaSe, CdSe, InSe, ZnTe, CdTe, GaTe, HgTe, GaSb, InSb, AlSb, CdS, GaN,and AlN; wherein the carrier gas is H₂, wherein the first precursor isHCl, the Group II/III element comprises at least one of Zn, Cd, Hg, Al,Ga, and In; and wherein the second precursor is one of AsH₃ (arsine),PH₃ (phosphine), H₂Se (hydrogen selenide), H₂Te (hydrogen telluride),SbH₃ (hydrogen antimonide), H₂S (hydrogen sulfide), and NH₃ (ammonia).The process may be an HVPE (hydride vapor phase epitaxy) process. Morethan one second precursor may be used, in varying ratios.

According to a first variation of the invention, the substrate is GaAs(gallium arsenide), the second precursor is PH₃ (phosphine), and theheteroepitaxial growth is GaP (gallium phosphide).

According to another variation of the invention, the substrate is GaP(gallium phosphide), the second precursor is AsH₃ (arsine), and theheteroepitaxial growth is GaAs (gallium arsenide).

According to a further variation of the invention, the substrate is GaAs(gallium arsenide), the second precursor is H₂Se (hydrogen selenide),and the heteroepitaxial growth is ZnSe (zinc selenide).

According to another variation of the invention, the substrate is ZnSe(zinc selenide), the second precursor is AsH₃ (arsine), and theheteroepitaxial growth is GaAs (gallium arsenide).

According to another variation of the invention, the substrate is GaSb(gallium antimonide), the second precursor is H₂Te (hydrogen telluride),and the heteroepitaxial growth is ZnTe (zinc telluride).

According to a further variation of the invention, the substrate is ZnTe(zinc telluride), the second precursor is SbH₃ (antimony trihydride,called also stibine), and the heteroepitaxial growth is GaSb (galliumantimonide).

According to a further variation of the invention, the substrate is GaN(gallium nitride), the second precursor is H₂Se (hydrogen selenide), andthe heteroepitaxial growth is β-GaSe (gallium selenide).

According to another variation of the invention, the substrate is β-GaSe(gallium selenide), the second precursor is NH₃ (ammonia), and theheteroepitaxial growth is GaN (gallium nitride).

According to a further variation of the invention, the substrate is GaP(gallium phosphide), the second precursor is H₂Se (hydrogen selenide),and the heteroepitaxial growth is α-GaSe (gallium selenide).

According to a further variation of the invention, the substrate isα-GaSe (gallium selenide), the second precursor is PH₃ (phosphine), andthe heteroepitaxial growth is GaP (gallium phosphide).

According to another variation of the invention, the substrate is AlN(aluminum nitride), the second precursor is H₂Se (hydrogen selenide),and the heteroepitaxial growth is β-GaSe (gallium selenide).

According to a further variation of the invention, the substrate isβ-GaSe (gallium selenide), the second precursor is NH₃ (ammonia), andthe heteroepitaxial growth is AlN (aluminum nitride).

According to a further variation of the invention, the substrate is GaAs(gallium arsenide), the second precursor is AsH₃ (arsine), and theheteroepitaxial growth is AlAs (aluminum arsenide).

According to another variation of the invention, the substrate is GaSb(gallium antimonide), the second precursor is AsH₃ (arsine), and theheteroepitaxial growth is InAs (indium arsenide).

According to another variation of the invention, the substrate is CdS(cadmium sulfide), the second precursor is PH₃ (phosphine), and theheteroepitaxial growth is InP (indium phosphide).

According to another variation of the invention, the substrate is InAs(indium arsenide), the second precursor is H₂Se (hydrogen selenide), andthe heteroepitaxial growth is CdSe (cadmium selenide).

According to another variation of the invention, the substrate is InSb(indium antimonide), the second precursor is H₂Te (hydrogen telluride),and the heteroepitaxial growth is CdTe (cadmium telluride).

According to another variation of the invention, the substrate is InP(cadmium telluride), the second precursor is H₂Te (hydrogen telluride),and the heteroepitaxial growth is GaTe (gallium telluride).

According to another variation of the invention, the substrate is CdTe(indium phosphide), the second precursor is H₂Te (hydrogen telluride),and the heteroepitaxial growth is HgTe (gallium telluride).

According to another variation of the invention, the substrate is InSb(indium antimonide), the second precursor is H₂Te (hydrogen telluride),and the heteroepitaxial growth is HgTe (gallium telluride).

According to another variation of the invention, the substrate is GaSb(gallium antimonide), the second precursor is SbH₃ (antimonytrihydride), and the heteroepitaxial growth is AlSb (aluminumantimonide).

According to another variation of the invention, the substrate is CdTe(cadmium telluride), the second precursor is SbH₃ (antimony trihydride),and the heteroepitaxial growth is InSb (indium antimonide).

According to another variation of the invention, the substrate is InAs(indium arsenide), the second precursor is SbH₃ (antimony trihydride),and the heteroepitaxial growth is GaSb (gallium antimonide).

According to another variation of the invention, the substrate is InAs(indium arsenide), the second precursor is SbH₃ (antimony trihydride),and the heteroepitaxial growth is AlSb (aluminum antimonide).

According to another variation of the invention, the substrate is InP(indium phosphide), the second precursor is H₂S (hydrogen sulfide), andthe heteroepitaxial growth is CdS (cadmium sulfide).

Details regarding other heteroepitaxial cases can be found in FIG. 13.

Additional objects, advantages, and novel features of the invention willbe set forth in part in the description which follows, and in part willbecome apparent to those skilled in the art upon examination of thefollowing or may be learned by practice of the invention. The objectsand advantages of the invention may be realized and attained by means ofthe instrumentalities and combinations particularly pointed out in theappended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of this specification, illustrate embodiments of the presentinvention and, together with a general description of the inventiongiven above, and the detailed description of the embodiments givenbelow, serve to explain the principles of the present invention. Thepatent or application file contains at least one drawing executed incolor. Copies of this patent or patent application publication withcolor drawing(s) will be provided by the Office upon request and paymentof the necessary fee.

FIG. 1 illustrates the energy gap as a function of the lattice constantsfor some traditional compound semiconductor materials;

FIG. 2A illustrates that the biaxial strain a can be resolved into auniaxial shear stress τ on the {111} dislocation glide plane;

FIG. 2B presents a TEM image of the strain being relaxed in a plastic(dislocation nucleation) process during GaP/GaAs heteroepitaxy;

FIG. 3A presents an SEM image of a GaAs substrate preheated for 1 h inAsH₃+H₂ atmosphere, according to an embodiment of the invention;

FIG. 3B presents an SEM image of a GaAs substrate preheated in H₂atmosphere only and shows the damages in result of the thermaldecomposition of the GaAs surface.

FIG. 3C presents a SEM image with a higher magnification of a GaAssubstrate preheated for 1 h in PH₃:H₂ atmosphere.

FIG. 3D presents an AFM images of a small area from the SEM image shownon FIG. 3C;

FIG. 4A presents an AFM image at lower magnification than FIGS. 3A-3D ofa GaAs substrate preheated for 1 h in PH₃:H₂ atmosphere, according to anembodiment of the invention;

FIG. 4B presents an AFM image at lower magnification of a GaP substratepreheated for 1 h in AsH₃:H₂ atmosphere, according to an embodiment ofthe invention;

FIG. 4C presents an AFM image of a small area of FIG. 4A at a highermagnification of a GaAs substrate preheated for 1 h in PH₃:H₂atmosphere, according to an embodiment of the invention;

FIG. 4D presents an AFM image of a small area of FIG. 4B at a highermagnification of a GaP substrate preheated for 1 h in AsH₃:H₂atmosphere, according to an embodiment of the invention;

FIG. 5 presents the thermal decomposition of AsH₃ and PH₃ as a functionof the temperature in a quartz vessel with (the two curves at left) andwithout (the two curves at right) the presence of a GaAs (with arsine)or a GaP (with phosphine) substrates within the vessel;

FIG. 6A presents an SEM image of a GaAs substrate preheated in PH₃:H₂atmosphere;

FIGS. 6B-6C present an EDS analysis that showing the presence of onlyGaAs in the pitted (dark) spots; in contrast the partly covered (white)areas indicate the presence of GaAsP;

FIG. 7A presents a Nomarski cross section image that shows the formationof an intermediate layer between the GaAs substrate and the growing GaPlayer;

FIG. 7B presents an EDS profile analysis that indicates that the formingbuffer layer is a GaAsP ternary with a gradually changing composition;

FIG. 8 presents a Nomarski cross section image of a grown multilayerheterostructure first of GaAs/GaAs and after that of GaP/GaAs;

FIGS. 9A, 9B, 9C, 9D, and 9E_present SEM top surface images of an OPGaAstemplate showing the adjacent area around the boundary between twooppositely oriented domains: (FIG. 9A) Before preheating; (FIG. 9B);After preheating in PH₃:H₂ atmosphere; (FIGS. 9C and 9D); Areas from(FIG. 9B) present an increasing magnification, showing how thelongitudinally-shaped pits in the opposite domains are oriented in twomutually perpendicular directions; and (FIG. 9E) presents a Nomarskicross section image of a 138 μm thick OPGaP grown heteroepitaxially onan OPGaAs template, according to an embodiment of the invention;

FIG. 10A illustrates a schematic drawing of an HVPE apparatus foroptimized thick heteroepitaxial growth of GaP with in-situ pre-growthtreatment of the GaAs substrate or the template, according to anembodiment of the invention;

FIG. 10B illustrates a schematic drawing of an HVPE apparatus foroptimized thick heteroepitaxial growth of GaAs with in-situ pre-growthtreatment of the GaP substrate or the template, according to anembodiment of the invention;

FIG. 10C illustrates a schematic drawing of an HVPE apparatus foroptimized thick heteroepitaxial growth of ZnSe with in-situ pre-growthtreatment of the GaAs substrate or the template, according to anembodiment of the invention;

FIG. 10D illustrates a schematic drawing of an HVPE apparatus foroptimized thick heteroepitaxial growth of ZnTe with in-situ pre-growthtreatment of the GaSb substrate or the template, according to anembodiment of the invention;

FIG. 10E illustrates a schematic drawing of an HVPE apparatus foroptimized thick heteroepitaxial growth of GaSb with in-situ pre-growthtreatment of the ZnTe substrate or the template, according to anembodiment of the invention;

FIG. 10F illustrates a schematic drawing of an HVPE apparatus foroptimized thick heteroepitaxial growth of hexagonal β-GaSe with in-situpre-growth treatment of the GaN (or AlN) substrate or the template, orzinc blended α-GaSe with in-situ pre-growth treatment of the GaPsubstrate or the template, according to an embodiment of the invention;

FIG. 10G illustrates a schematic drawing of an HVPE apparatus foroptimized thick heteroepitaxial growth of GaN (or AlN) with in-situpre-growth treatment of the hexagonal β-GaSe substrate or the template,or zinc blended GaP with in-situ pre-growth treatment of the α-GaSesubstrate or the template, according to an embodiment of the invention;

FIG. 10H illustrates a schematic drawing of an HVPE apparatus foroptimized thick heteroepitaxial growth of GaAs with in-situ pre-growthtreatment of the ZnSe substrate or the template, according to anembodiment of the invention;

FIG. 10I illustrates a schematic drawing of an HVPE apparatus foroptimized thick heteroepitaxial growth of AlAs with in-situ pre-growthtreatment of the GaAs substrate or the template, according to anembodiment of the invention;

FIG. 10J illustrates a schematic drawing of an HVPE apparatus foroptimized thick heteroepitaxial growth of InAs with in-situ pre-growthtreatment of the GaSb substrate or the template, according to anembodiment of the invention;

FIG. 11 depicts the thickness of the pseudomorphous growth (the criticalthickness h_(c)) expressed as the number of monoatomic layers as afunction of the lattice mismatch;

FIGS. 12A, 12B, and 12C depict TEM cross sectional images withincreasing magnification showing rough surface interface between thegrowing GaP layer and the GaAs substrate;

FIG. 12D depicts that distortions of different kind, such as creating ofvoids or roughening of surfaces near the interface make difficult todetermine the thickness of the pseudomorphous growth h_(c) and theperiodicity τ of the MDs; and

FIG. 13 presents a number of suitable heteroepitaxial cases.

It should be understood that the appended drawings are not necessarilyto scale, presenting a somewhat simplified representation of variousfeatures illustrative of the basic principles of the invention. Thespecific design features of the sequence of operations as disclosedherein, including, for example, specific dimensions, orientations,locations, and shapes of various illustrated components, will bedetermined in part by the particular intended application and useenvironment. Certain features of the illustrated embodiments have beenenlarged or distorted relative to others to facilitate visualization andclear understanding. In particular, thin features may be thickened, forexample, for clarity or illustration.

DETAILED DESCRIPTION OF THE INVENTION

A process for thick heteroepitaxial growth of semiconductor materials ispresented below. The semiconductor structures of the growing layer(s)may be deposited in a horizontal or vertical direction, on plainsubstrates or on patterned templates, including orientation-patternedtemplates.

A further embodiment of the invention states that the heteroepitaxialgrowth is preceded by an in-situ pre-growth treatment of the substrateor the template followed by at least 300-500 μm thick heteroepitaxialgrowth of one or more doped or undoped semiconductor materials or theirbinaries, ternaries, or quaternaries.

In order to be successful, each separate heteroepitaxial combinationmust meet particular requirements, including the lattice mismatch andthe accumulated strain. Accordingly, the heteroepitaxy exhibited in anyparticular example below will be based on the degree and the sign of thelattice mismatch between substrate and growing layer, as well as on howthe strain accumulated in the growing layer as a result of the latticeand thermal mismatch is released—in plastic or elastic strain releaseprocesses. An example of such a process is the periodic formation of theso-called “misfit dislocations” (MDs). Such dislocations may appearafter a certain critical thickness h_(c) of the so-called“pseudomorphous growth” during which the layer is “forced” to grow withthe lattice constant of the substrate. (Note: The words “mismatch” and“misfit” are almost identical, but their meanings are distinct in thiscontext. However, in particular cases, one will be preferred to theother. For example, it is proper to say “lattice mismatch” and “misfitdislocations”.) In this invention we teach that the sign and the degreeof the lattice mismatch and the periodicity of the MDs may be used ascriteria for one to determine how successful a new case of heteroepitaxycould be.

The lattice mismatch f % is calculated using the formula:f %=(b ₀ −a ₀)/a ₀ 0.100  (eq. 1)where a₀ and b₀ are the lattice constants of the substrate and the layermaterial. The lattice constants of some traditional semiconductormaterials are presented in FIG. 1, which shows their bandgap energies asa function of their lattice constants. From FIG. 1, one can easily seethat some materials, although having different bandgap energies, haveclose lattice constants and could be suitable for heteroepitaxy. Suchare, for example, GaP and Si, GaAs and ZnSe, AlAs and GaAs, etc.

The periodicity r of the misfit dislocations is determined by:τ=100/f %  (eq. 2)

As an example, we can determine the lattice mismatch f % and theperiodicity of the MDs τ in the particular heteroepitaxial case ofgrowth of GaP on a GaAs substrate. The lattice constant (a₀) ofGaAs=5.6532 Å, while the lattice constant (b₀) of GaP=5.4512 Å.According to equations (1) and (2), in this example the lattice mismatch(f_(a)) is negative (−3.57%) because b₀ GaP<a₀ GaAs; we should expectthe appearance of MDs at a periodicity τ of about 28 (i.e. 100/3.57)interatomic distances.

Such lattice mismatch (−3.57%) may be considered as large. Latticemismatches of 3-4% and more are, in general, considered as relativelylarge, while lattice mismatches under 1% may be considered as relativelysmall. Thus the lattice mismatch between GaP and GaAs (−3.57%) may beconsidered as large, while the lattice mismatch between ZnSe and GaAs(+0.26), for example, may be considered as small. However, whether aparticular mismatch can be considered as large or small depends on otherfactors as well, for example, on the strength of the bonds (the bonddissociation energies)—between the atoms of the substrate and those ofthe growing layer. For example (Table 1) the bond energies of the bondsGa—As and Ga—P are in the same order of magnitude, which means that theAs and P atoms can easily replace each other. That is why GaAs and GaPsubstrates can be pre-treated easily—thus they can easily grow one onanother. As Table 1 also shows, in this point of view GaSb, InSb and InPare also “easy” substrates due to the low bonding energies of theiratoms. However, as one can see from Table 1, due to the high bondingenergy of the Ge—Ge bonds and, especially, of the Si—Si bonds, thesecommon substrates are hardly treatable. Thus, in the case of growth ofGe/Se, for example, namely because the atomic bonds are strong in bothmaterials, the lattice mismatch of +3.96% between Ge and Si isconsidered as huge, no matter that as a number this mismatch is not muchdifferent from the lattice mismatch between GaAs and GaP:

TABLE 1 Some bond dissociation energies (standard state enthalpychanges) at T = 298K. Bond dissociation energy Bond ΔHf₂₉₈ [kJ/mol]Ga—As 209 Ga—P 230 Ga—Sb 209 In—Sb 152 In—P 198 Ge—Ge 274 Si—Si 327

The sign of the lattice mismatch, minus (−) or plus (+), is alsoimportant. For example, it was determined that the thickness of thepseudomorphous growth, i.e. the critical thickness h_(c), is larger whenthe lattice mismatch is negative and the film is growing during thepseudomorphous growth under tensile strain (because the layer has asmaller lattice constant than the substrate) than in the case of apositive lattice mismatch when the layer is growing compressivelystrained. The significant difference in the mechanisms of dislocationnucleation (after the moment in which the pseudomorphous growth becomesenergetically unfavorable and the accumulated elastic strain must berelieved somehow) in the case of tension vs. compression contributes tothis difference as well. For example, while in the compression case thedislocations nucleate by squeezing out an atom at the base of surfacedepressions, in the tension case, the nucleation of misfit dislocationsinvolves the concerted motion of a relatively large number of atoms,leading to insertion of an extra lattice (plane) row into an alreadycontinuous film. In addition to all that the film morphology dependsintimately on the sign of the misfit (+ or −), i.e., on type of thestrain (tensile or compressive). It is experimentally confirmed, forexample, that growth under tensile strain (negative misfit) favors 2Dgrowth which usually results in smooth surface morphology, whilecompressive growth facilitates 3D growth which typically results inhillock type, rougher surface morphology. This supports again the notionthat the growth under tensile strain, as it is in the case of growth ofGaP on GaAs, should be more favorable than, for example, the oppositecase of growth of GaAs on GaP, or in other cases with positive misfits.

The linearly-increasing elastic strain accumulated during thepseudomorphous growth must be released at a certain point. The formationof MDs is one of the possible mechanisms of strain relief. However,deeper crystallographic considerations are necessary to determine wherethey should be expected, or on which crystallographic plane it is mostprobable for the MDs to appear. In a zinc blended structure (i.e. thisis the structure of many of the compound semiconductor materialspresented herein, e.g. GaAs, GaP, ZnSe, etc.), for example, the biaxialstrain a accumulated during pseudomorphous growth can be resolved (seeFIG. 2A) into a uniaxial shear stress τ on the (111) dislocation glideplane. At the same time, the directions of σ and τ, of course, will beopposite depending on whether the layer is growing under tensile(negative mismatch) or compressive (positive mismatch) strain. In otherwords, whether the lattice constant of the growing layer is smaller orlarger than the lattice constant of the substrate. Thus, recalling thatthe accumulated strain may be released in either elastic (surfaceroughening) or plastic (dislocation nucleation) relaxation processes,one may assume that in the latter case the most probable plane where themisfit dislocations shall appear is (111). Such dislocations were indeednoticed in the (111) zone in the particular case of GaP/GaAsheteroepitaxy as the TEM image in FIG. 2B indicates.

On the other hand, other distortions such as the formation of voids orthe roughening of surfaces, for example roughening of the growinginterface, may also contribute in absorbing the accumulated elasticenergy as alternative strain relief mechanisms that may postpone theformation of the MDs until a later stage of growth. All this providedsome insight that it might not be terribly detrimental if the surface ofthe substrate was to be made rougher with the intention of facilitatingthe initial layer's nucleation.

Roughening of the surface may be the result of exposing the substrate toa non-native precursor during the preheating stage. This is supported byFIGS. 3C-3D which present an SEM image (FIG. 3C) and an AFM image (FIG.3D) from the image on FIG. 3C of a GaAs substrate preheated for 1 h in aPH₃:H₂ atmosphere. The damages (not shown) are about the same afterpreheating the GaAs substrate for 1 hour in AsH₃:H₂ atmosphere, andexposed after that to PH₃:H₂ for only 1 minute).

FIG. 3A presents an SEM image of a GaAs substrate preheated for 1 h inAsH₃+H₂ atmosphere, according to an embodiment of the invention; one cansee that such roughening (see FIGS. 3C-3D) never occurs when thesubstrate is exposed to its native precursor (see FIG. 3A) during thepreheating stage. However, although this temperature is still lower thanthe deposition temperature, some thermal decomposition of the substratemay occur if the GaAs substrate is preheated only in an H₂ atmosphere,as shown in FIG. 3B. For example, to prevent the thermal decompositionduring growths of GaAs or GaP the wafers should to be exposed to anAsH₃:H₂ (the GaAs wafer) or a PH₃:H₂ (the GaP wafer) mixture once thefurnace temperature achieves 350-400° C., even though the decompositiontemperature is much higher—about 700° C. Preheating the substrate onlyin H₂, for example, actually results in an intensive evaporation (andshortage) of the more volatile group V-atom (As or P in the particularcases of GaAs or GaP) (see FIG. 3B). Going further in this direction bypretreating the GaAs substrate in phosphine (PH₃), or in a PH₃:H₂mixture, leads to severe pitting of the GaAs surface (see FIGS. 3C, 3D,4A, and 4C). Similarly, preheating the GaP substrate in AsH₃ or in anAsH₃:H₂ mixture resulted even in more severe pitting of the GaP surface(see FIG. 4B and FIG. 4D). Even with the time of exposure as short as 1minute, the visible surface damages on the substrate's surface will beabout the same as after one hour of exposure of the substrate to thenonnative precursor (as shown in FIGS. 3C-3D and 4A-4D).

It is thought that the stronger influence of AsH₃ on GaP than of PH₃ onGaAs (compare the ranges of the distances between peaks and valleys onthe scales that are left from FIG. 4A and FIG. 4B) should be attributedto the lower surface quality (i.e. the higher etch pit density) of GaP,which should be more vulnerable to the AsH₃ attack. However, to fairlycompare this effect we must also consider the differences in the thermaldecomposition of GaAs and GaP as well as the differences in the thermaldecomposition of AsH₃ and PH₃. It is known, for example, that theAs-vapor pressure over a GaAs surface is lower in comparison to theP-vapor pressure over GaP, which means that GaP decomposes faster or toa greater extent than GaAs. At the same time, the thermal decompositionof PH₃ and AsH₃ over a GaP substrate or, respectively, over a GaAssubstrate in the temperature range of 400-600° C., (see left side ofFIG. 5), are nearly equivalent, but still slightly in favor of the AsH₃decomposition under 500° C. However, according to the right side of FIG.5, the AsH₃ decomposition over a quartz surface (the surface of ourreactor tube) within the pretreatment temperature range, i.e. above 350°C. but less than the growth temperature, is significantly stronger thanthe thermal decomposition of PH₃. This means that with increasingtemperature, in one example, we provoke higher (in comparison to thecase of GaAs) losses of P-atoms from the GaP surface. However, inanother example, during the preheating stage, the decomposition of theAsH₃, which is flowing over the GaP surface inside the reactor tube,seems stronger than the thermal decomposition of PH₃—it is at leaststrong enough to provide plenty of As-atoms that can occupy the P-sitesin the GaP crystal cell of the hot GaP substrate, which are liberateddue to the GaP decomposition. Thus, by attacking the surface, AsH₃ leadsto more severe pitting of the GaP surface (see FIG. 4B) than PH₃ on theGaAs surface (see FIG. 4A).

It was discovered that roughening the surface is not the only result ofexposing the semiconductor material to a non-native precursor during thepreheating stage. Elemental analysis performed by Electron DispersionSpectroscopy (EDS) of surfaces exposed to non-native precursors (seeFIGS. 6A-6C) indicated that the exposing of GaAs to PH₃ or to theaforementioned PH₃-containing mixtures (PH₃:H₂ or AsH₃:PH₃:H₂) provokesthe start of the formation of ternary GaAs_(x)P_(1-x) islands stillduring the preheating stage prior to initiating the very heteroepitaxialgrowth of GaP/GaAs. These islands eventually coalesce forming acontinuous intermediate transition GaAs_(x)P_(1-x) buffer layer (FIG. 7Aand FIG. 7B). By absorbing some of the mismatch strain, this transitionbuffer layer helps to accommodate the growing layer to the foreignsubstrate, realizing a smooth transition between the substrate to thegrowing material.

The significance of the growth of non-native precursors onsemiconductors is not a single isolated case but works with morematerials (see FIG. 4A and FIG. 4B), and teaches us that we can growmultilayered heterostructures (see FIG. 8) in which two or morematerials can by alternated multiple times in the same grown structure.For example, we can start growing GaP on a GaAs substrate pretreated inphosphine (or in a PH₃:H₂ mixture), and after that, pretreat the alreadygrown HVPE GaP layer in arsine (or in an AsH₃:H₂ mixture) and grow GaAs,and after that to pretreat the already grown HVPE GaAs layer in, forexample, hydrogen selenide (H₂Se) (or H₂Se:H₂) following by growth ofZnSe/GaAs. It is not difficult to imagine that such structures may haveapplications that will cross known boundaries. For example, thecombination of several different semiconductor materials in amulti-material structure may be used for various device developments,including in the solar cell industry for portable high power solar cellsor for multicolor detectors that simultaneously cover large areas of thespectrum, in particular the two atmospheric windows of transparencybetween 2-5 and 8-12 μm, or for development of phase or quasi-phasematching (QPM) structures for frequency conversion of laser sourcesoperating in the poorly covered mid and longwave infrared. Finally,involving common substrates such as the unique electronic material Simay open the doors for numerous applications in optoelectronics.

The proposed approach is to some extent universal because it may beapplied to different materials deposited one over another in a one-stepepitaxial process, with or without the intentional assistance of anintermediate sub-lattice transition buffer layer (see FIG. 7) depositedbetween them. Such a layer, indeed, facilitates the growth of thefollowing heterostructure. For example, one may need to control thequality of the buffer layer by carefully selecting when, i.e. at whichtemperature, to introduce the non-native substrate into the mixture. Onemay also gradually change the ratio of the non-native/native precursorgasses (i.e. the second precursors) in the mixture in order gradually tochange the x composition of the starting material from a pure substratematerial and ending up with a pure layer material. As one example, thegrowth of GaP on GaAs, the process may be started by maintaining theGaAs substrate initially in an AsH₃+H₂ mixture in order to protect itssurface from thermal decomposition. Later, we may gradually startintroducing PH₃, and increasing its amount in the mixture AsH₃:PH₃:H₂,while gradually reducing the amount of AsH₃ to zero. This processprovides the conditions for growing the desired intermediate transitionGaAs_(x)P_(1-x) ternary in which the composition changes gradually toachieve a balanced growth of a pure GaP layer. The disclosed processallows precise control of both the composition and the thickness of thebuffer layer that helps to achieve a smooth transition between thesubstrate and the growing material. In contrast with other well-knownapproaches, the proposed process for buffer layer deposition is anin-situ step from a continuous growth process: This naturallyincorporates in one process all major techniques for engineering ofbuffer layers such as: (i) gradual change of the layer composition; (ii)growth on patterned templates which at larger lattice mismatches aims toallow a more uniform nucleation—in our case this is a growth on a randompattern of ternary islands; and (iii) using the buffer layer forefficient strain release, etc.

Because it is capable of controlling the thickness of the buffer layer,the invention allows one to extend the idea of the deposition of aternary transition buffer layer to the growth of ternaries hundreds ofmicrons thick. This can be achieved by maintaining a constant ratio inthe mixture between the native and the non-native precursor that willensure the desired composition (x) in the growing ternary during theentire growth process. As an example, the disclosed process has beenused to grow GaAs_(x)P_(1-x) ternary layers up to 300 μm thick on bothGaAs and GaP substrates at the relatively high growth rate of about 100μm/h. The ability to grow thick ternaries by the proposed process isimportant because, the tailoring of different compositions allows one toachieve the best combination of properties which are suitable for aparticular application. For example, it was discovered that in theparticular case of GaAs_(x)P_(1-x) the following compositionGaAs_(0.34)P_(0.66) (x=0.34) provides lower two-photon absorption (2PA)than of GaAs but higher nonlinear susceptibility than of GaP in thewavelength of interest (1-1.7 μm)—these parameters are of greatimportance for applications such as QPM frequency conversion.

Thus such a combination of material properties satisfies requirementsfor using this new ternary material for frequency conversion devices. Atthe same time ternaries may be grown on either parenting material (inthis case on GaAs or on GaP substrates) depending on how close to thegiven substrate is to their composition.

Such combinations of materials also, in general, represent strongerheteroepitaxial cases having smaller lattice and thermal mismatches withthe related substrate.

The growth of ternaries is also an easier heteroepitaxial task becauseof the expected smaller lattice and thermal mismatches of the ternary(in this case GaAs_(x)P_(1-x)) with these two substrates (i.e. GaAs andGaP), compared to the lattice mismatch between the original substrateand growing materials (GaP and GaAs). Modification of materialproperties is another opportunity that the proposed invention provides.The aforementioned example demonstrates the usefulness of suchmodifications for the development of new laser sources. However, othermaterials will provide thick growths of other ternaries or quaternariesthat may result in products that could support other research anddevelopment fields—optoelectronics, sensing (detectors), solar cellsindustry, etc.

This invention also allows one to use the pre-growth treatment approachfor better polarity control during both the fabrication oforientation-patterned (OP) templates and the following thick HVPE growthon them. In contrast to the case where a regular substrate is exposed toa non-native precursor and the shape of the pits are irregular andrandomly distributed, in the case of treating an OP template, the shapeof the etch pits are rather longitudinal and oriented in two mutuallyperpendicular directions on the surface of domains with oppositecrystallographic orientations (opposite polarity).

FIG. 9A and FIG. 9B, for example, present an area around the boundarybetween two oppositely-oriented domains, (100) and (100), of an OPGaAstemplate before and after preheating the OPGaAs template in the presenceof PH₃. FIG. 9C and FIG. 9D present the outlined small areas from FIG.9B at higher magnifications. As one can see in FIG. 9C and FIG. 9D,after exposing the template to PH₃, the pits on the treated OPGaAstemplate are longitudinal and tend to be oriented in two mutuallyperpendicular directions, along [01 1 ] and along [011], on the (100)and ( 1 00) oriented domains. This phenomenon, from a practical point ofview, is an important feature of this invention, because it gives us atool to easily determine the presence of the opposite domainorientations on the OP-template. Moreover, this approach is also an easyway to estimate the quality of the OP-template prior to growth, or thequality of the grown QPM structure, which is a simple in-situnon-destructive step. Until now it was possible to determinecrystallographic orientations only with great uncertainty: (i) by theshape of specific surface features, which are more inherent to aparticular orientation, or (ii) by etching, i.e. damaging the surface,after which the surface is not acceptable for subsequent growth.Heteroepitaxial growth, performed on such OPGaAs templates, resulted inthick (up to 300 μm) high quality OPGaP with rectangular domains havingexcellent fidelity (see FIG. 9E). Such types of templates may be usedfor QPM frequency conversion devices that could radiate, practically, inthe entire spectrum, covering uncovered or poorly covered frequencyranges. Based on III-Nitrides, for example, such devices can radiatepartly in the UV and partly in the close to the UV visible spectrum,making them useful for water purification, LEDs, energy conservation,high power lasers for high energy physics, etc. Frequency conversiondevices based on wideband compound semiconductors such as GaAs, GaP,ZnSe, or ZnTe, radiating in the mid- and long-wave IR (especially ifthey cover both atmospheric windows of transparency) may be useful in IRcountermeasures for protection of military aircraft and ships from heatseeking or laser guided missiles. Respectively, FC sources radiating inthe microwave and/or THz regions may be useful in airport scanners,remote sensing of chemicals (explosives) or biological agents, inindustry, medicine (biopsy-free cancer cell detection), and science (THzspectroscopy).

Depending on which mode of heteroepitaxy is stimulated during growth,e.g. Volmer-Weber (island growth), Frank-van der Merwe (layer-by-layergrowth) or Stranski-Krastanov (layer-plus-island growth), and what typeof strain relief is present, i.e. elastic (surface roughening) orplastic (dislocation formation), the proposed process may be used forgenerating quantum wells or quantum dots, or other nano- andmicro-structures. By skillful use of these growth modes this process maycombine similar or different materials, for example electronicmaterials, such as Si with optoelectronic materials, e.g. GaAs, GaP,ZnSe, ZnTe, etc., or even their ternaries or quaternaries. Thus thedescribed approach contributes to advances in the development ofoptoelectronic devices as well.

The disclosed process allows for one-step thick growth without the needfor a preliminary deposition (usually using a thin growth technique) ofanother material with a smaller lattice mismatch with the substrate. Forexample, as it is known in the prior art, thick HVPE GaN layers can begrown on SiC substrates only after the deposition of a thin AlN layer byMBE or MOCVD. Similarly, thick HVPE GaN may be grown on thin GaN or AlNdeposited in advance on sapphire, again by MOCVD or MBE. The MBE andMOCVD from one side and the HVPE from another are growth processes whichare quite different by their natures, as the first two (MOCVD and MBE)are far-from-equilibrium processes and can be used only for up to 1-2 μmthin growth, while the third one (HVPE) is a close-to-equilibriumprocess that can be used for thick growth. This means that the oldapproach, using MOCVD or MBE, is a two-step growth process, which needsmore high-tech instruments, i.e. more investments. These limitations arenot necessary with the disclosed approach.

The disclosed process also eliminates the need to first grow (typicallyby HVPE) an intentionally-deposited low temperature (LT) buffer layer onthe substrate prior to the growth of the high temperature (HT) layer.This is known the prior art for the thick HVPE growth of GaN, forexample. The deposition of an intermediate layer aims to reduce thestrain between the sapphire substrate and the growing GaN layer. A LTbuffer layer does this job but, at the same time, it is a layer withextremely low crystalline quality and, thus, it is a source of a greatnumber of dislocations, i.e. the LT buffer layer does not provide alwaysan optimal foundation to start the growth of the actual GaN layer. Theproposed approach allows the formation of an intermediate transitionlayer naturally, in-situ, and during the initial preheating of thesubstrate and not through a growth process—this occurs prior to thestart of growing the actual layer but also may continue during theinitial stages of growth as well. According to the present invention, itis not necessary for the buffer layer to be a LT layer. Instead, thechoice of the temperature of the buffer layer formation may becontrolled, and thus its quality may be controlled.

We would like once again to emphasize another significant differencebetween the proposed buffer layer growth and the prior art by using thesame example: an HVPE-grown LT GaN buffer layer. In the prior art caseone can choose to change the V-III ratio during the buffer layerdeposition in order for this layer to accommodate the substrate andgrowing layer. However, the buffer layer and the subsequent layer arefrom still from one and the same material, GaN. Thus there is stillhomoepitaxy at the boundary between LT GaN buffer layer and HT growingGaN layer. In contrast, the inventive method disclosed herein is aheteroepitaxial case, and we may selectively change the composition ofthe growing material for a gradual replacement of the substrate materialwith the growing layer.

Thus the optimized growth approach presented herein avoids or solvesmany of the current problems and shortcomings of heteroepitaxy. Theoptimized growth approach presented herein demonstrates severaladvantages over the known and comfortable homoepitaxy processes. Thedisclosed process clearly indicates that there are particular cases whenheteroepitaxy is preferable even when homoepitaxy is possible.

Heteroepitaxy provides economic and quality advantages: for example, theavailable GaP (for 2-inch wafers) on the market is 5-6 times moreexpensive than the corresponding GaAs, the GaP having much lower qualitywith respect to the etch pit density (EPD) and wafer parallelism. Thismeans that the quality of OPGaP templates prepared from such wafers willbe also low, because we should expect poor quality of the HVPE growth onthem. Accordingly, the performance of frequency conversion devices basedon such templates will also be unacceptable. The ability to use GaAssubstrates and OPGaAs templates for growth of GaP and OPGaAs solves thisproblem.

In addition, heteroepitaxy enables the use of techniques suitable forthick epitaxial growth, e.g. HVPE, and the corresponding particulartechnological applications that require thick epitaxial growths. At thismoment, due to the complex growth mechanisms of heteroepitaxy, knowledgeof such mechanisms is relatively limited despite the great deal ofeffort made over the last couple of decades. For example, on an atomicscale it is known how the growth proceeds only for the first fewmonoatomic layers, even for the homoepitaxial growth of plainsemiconductors, e.g. Si and Ge. That is why the semiconductor industryhas adopted primarily thin growth techniques such as MOCVD and MBE, andonly for a limited number of well-studied materials.

Heteroepitaxy allows for the optimization of the most promisingcandidate for thick epitaxial growth, the HVPE technique. Othertechniques for thicker epitaxial growth used today are either too slow,resulting in impractically small crystals (solvothermal growth, etc.),or thin layers (liquid phase epitaxy, etc.) or the nature of the growthtechnique is such that it does not allow precise control of the processparameters during growth. An example of such a process is physical vapordeposition (PVD) which is known mostly as a bulk growth technique(including for growth of ZnSe), although it was once used inheteroepitaxy of ZnSe/GaAs. However, the bulk growth was incapable ofproviding large enough single grain crystalline substrates, while thelayer growth led to rather low material quality and limited opticalresults. However, even though the HVPE approach provides more optionsfor control of the growth process and more choices for growth chemistry,HVPE also has its own problems. Such as, for example, the severecompetitive parasitic nucleation on the inner quartz surfaces of thereactor that accompanies the deposition: such parasitic nucleation onlyslows down the process, depletes the precursor sources, and deteriorateslayer quality.

The disclosed process allows successful heteroepitaxial growth even atrelatively large lattice and thermal mismatches, and without patterningthe substrate, which in many cases is the standard procedure. Thepresent invention is based on the accumulation of a significant amountof information about a great number of semiconductor materials andvarious growth processes used to grow them. This allowed us to realizethe significance of the determination of important characteristics ofheteroepitaxy, e.g. the thickness of the pseudomorphous growth, theperiodicity of the misfit dislocations, or the mechanisms of strainrelief, to make the related efforts in several particular cases. Inturn, the determination of these parameters allowed us to developcriteria by which to predict other successful cases of heteroepitaxy,and thus to realize heteroepitaxial growth at mismatches that at firstsight looked impossible.

The disclosed invention is based on our understanding of the complexchemistry and growth mechanisms of heteroepitaxy of widebandsemiconductor materials. The invention secures a smooth transitionbetween two materials, for example GaAs and GaP, not through aforced-growth process but during the preheating of the substrate andduring the initial stages of growth. During the initial growth stages,the process directs the gradual replacement of substrate atoms, forexample V-group atoms, with V-group atoms from the growing material. Forexample, the process may direct the replacement of As atoms in thecrystal cell of a GaAs substrate with P atoms during the preheatingprocess, which may be conducted in a phosphine (PH₃) atmosphere or amixture of phosphine (PH₃) and arsine (AsH₃). The process requires theuser to decide at which temperature (if still in the preheating stage)to initiate such replacement, and whether this temperature should bekept constant during the deposition of this buffer layer, or whether itshould be increased at some rate until achieving the growth temperature.The user also must decide whether to keep the arsine/phosphine ratioconstant or to gradually change this ratio from arsine only to phosphineonly in order to achieve the smoothest transition between substratematerial (e.g. GaAs) and grown layer material (e.g. GaP).

From another point of view the disclosed approach is universal alsobecause it may be applied to a wide range of different materials havingwide ranges of differences in their lattice constants and their thermalexpansion coefficients (resp. different thermal conductivities)deposited one over another in a one-step epitaxial growth process, withor without the intentional deposition of the aforementioned intermediatesub-lattice transition buffer layer between them. This approach isdemonstrated to be successfully applied not only for growth of GaP onGaAs but also for the opposite growth of GaAs on GaP. These cases, fromthe perspective of lattice mismatches, are not highly favorable (seeFIG. 1). Accordingly, that is why the disclosed method may be applied toother related materials systems, some of which may have latticemismatches less unfavorable than GaP/GaAs. Such are, for example,ZnSe/GaAs or GaP/Si, or ZnTe/GaSb, etc. (see FIG. 1 and Table 2).

Regarding the particular case of the growth of ZnSe on GaAs, the GaAssubstrate (or template) is preheated in hydrogen selenide (H₂Se) mixedwith H₂, or in a H₂Se:AsH₃:H₂ mixture in order to partially andgradually replace the V-group atom (As) in the GaAs crystal cell with Seand thus form a GaAs_(x)Se_(1-x) ternary buffer layer. After this step,the growth may continue with the introduction of the Zn-precursor, whichmay be either metallic Zn overflowed by HCl (or an HCl+H₂ mixture) toform zinc chloride (ZnCl₂), or simply ZnCl₂ overflowed by H₂, or even aZn-rich ZnCl₂ solution overflowed by HCl+H₂ mixture. This may be thebetter choice due to the relatively high vapor pressure of zinc. Table 2compares the realized GaP/GaAs heteroepitaxy with some other examples(incl. ZnSe/GaAs) for prospective heteroepitaxial cases (more examplesare provided in FIG. 13).

TABLE 2 Some favorable cases of heteroepitaxy based on lattice mismatch.Heteroepitaxy Lattice mismatch [%] GaP on GaAs −3.57 ZnSe on GaAs +0.24ZnTe on GaSb +0.08 ZnTe on InAs +0.70 AlAs/GaAs +0.13 GaP/Si +0.37

From Table 2 one can easily see that all other examples provided aremore favorable than GaP/GaAs, providing much smaller (less than 1%)lattice mismatches. Of course, to make the “right” choice one shouldtake into account what the desired application might be and some otherrelated properties of the particular material candidate. For example, abrief comparison of ZnSe and ZnTe shows that ZnTe has about the sametransmission range as ZnSe but lower 2PA and 3 times higher nonlinearsusceptibility at the desired pumping wavelength of about 1 μm, i.e.ZnTe may be a better choice for nonlinear frequency conversion devices.Of course, one also should pay attention to the specific technologicallimitations related to the growth of a particular material, e.g. GaP maynot be grown by HVPE directly on Si, but it may be grown at high qualityby MOCVD or MBE. Nevertheless, the proposed technique may give betteroptions for many other materials, including some that have never beengrown epitaxially, or even by any other techniques, in a monocrystallineshape, and in a size large enough for practical use, e.g. ZnSe or ZnTe.

With regard to the GaP/Si combination (the last line in Table 2), whichis a growth of a compound semiconductor (GaP) on a plain semiconductor(Si), our approach may be slightly different due to the extremely strongSi—Si bond. In this case, it may be preferred to preheat the Sisubstrate in H₂ only to provoke some thermal decomposition (as shown inFIG. 3B) to allow some Si-atoms to escape the substrate and thus to givea chance for the approaching P-atoms to occupy their positions. In thealternative, one may attempt to do this by etching the Si-substrate witha known Si etchant. The opposite is also possible: the growth of Si onGaP, which is another case in which a solution for an etchant materialmay be required. For example, we may use silane (SiH₄) to “attack” theGaP substrate before we start growing Si on it.

These new suggested heteroepitaxial cases (Table 2 and FIG. 13) can beleveraged by their inclusion in related ternary or even quaternarymaterials that not only combine the best properties of two (or more)semiconductor materials (of course, in relation to the desiredparticular practical applications), but that also facilitate theheteroepitaxial growth disclosed herein, that, in general, the ternaryshall have a closer lattice match to the parenting substrate materials.Another direction that may be taken in developing this idea is to makethe lattice constants between substrate and growing layer closer bydoping the growing layer, while keeping in mind that doping with dopantsthat are different in size (i.e. smaller or larger ionic radius) willsignificantly change the lattice constant of the doped material. On theother hand, the dopant concentration may be gradually changed duringgrowth, which will form a transition layer with a gradually-changinglattice constant. This will secure an even smoother transition betweenthe substrate and the growing layer. Each of these variations of theproposed in-situ substrate pretreatment may be followed by a growthstage aimed to build thin or thick layers.

The following examples illustrate particular properties and advantagesof some of the embodiments of the present invention. Furthermore, theseare examples of reduction to practice of the present invention andconfirmation that the principles described in the present invention aretherefore valid but should not be construed as in any way limiting thescope of the invention.

Experimental

As illustrated in FIGS. 10A-10J, the related experiments were conductedin a hot wall 3-inch diameter horizontal quartz reactor 10 positioned ina three-zone resistive furnace 12. The furnace 12 is not depicted inFIGS. 10B-10J for clarity. If no ternaries are to be formed, the gasseslabelled as “ternary-forming gas” in FIGS. 10A-10J correspond to the“second precursor gas” of the claims. Mixtures of the second precursorgasses, in constant ratios or ratios varying over time, may be used tosupport the growth of ternaries, which may in turn support the growth ofthe desired growing layer. The sole “precursor gas” of FIGS. 10A-10Jcorresponds to the “first precursor gas” of the claims. The firstprecursor gas is usually hydrogen chloride (HCl) diluted to the desiredextent by the carrier gas (usually H₂). The role of the first precursorgas is to pick-up a II or III-group element (Ga, Al, Sb, Cd) from anopen boat or a bubbler, and with it to form a metal-chlorine compoundwhich is delivered to the mixing zone, making it available toparticipate in the growing process

The second precursor, or ternary-forming gas, is usually a hydride of aV or VI group element (AsH₃, PH₃, H₂S, SbH₃, etc.) diluted to thedesired extent by the carrier gas (usually H₂). The role of the secondprecursor (ternary-forming gas), which is actually the precursor of theV or VI-group element, is delivered to the mixing zone, making itavailable to participate in the growing process. We call the secondprecursor “ternary-forming” because the reactions between the firstprecursor gas and the ternary-forming gas on the foreign substrate mayresult in the formation of ternary islands on the substrate, which mayeventually coalescence to form a continuous ternary buffer layer.

Example 1—Growth of GaP on GaAs Substrate

These embodiments of the invention are based on hydride vapor phaseepitaxy (HVPE) and the heteroepitaxial growth of GaP on GaAs substrates,or the opposite case of the heteroepitaxial growth of GaAs on GaPsubstrates. However, one should bear in mind that these are twosuccessfully-realized examples, but not the most favorable examples interms of their lattice constants or in their thermal expansioncoefficients or thermal conductivities. As explained in the text above,many other semiconductor materials may be grown by this technique andare favored by the proposed approach. GaAs substrates andorientation-patterned GaAs templates (OPGaAs) are available at highquality and at a reasonable price. The lattice mismatch is larger butnegative, which means that the GaP layer grows under tensile stresswhich is the more favorable case because this arrangement compensatesthe naturally-compressed surface of the GaAs substrate.

With regard to FIG. 10A, a quartz boat 14 positioned in the first zone16 and filled with molten gallium (Ga) was placed in a one-inch diameterinner tube 18 and a mixture of hydrogen H₂ as carrier gas and HCl as afirst precursor gas was flowed over the boat 14. The purpose of the H₂carrier gas is not only to carry the HCl precursor gas but also todilute the HCl flow to a desired extent while the HCl flow picks up somegallium (Ga) from the boat 14 to form gallium chloride (GaCl₃) in thereaction:6HCl+2Ga→2GaCl₃+3H₂  (eq. 3)Another peripheral flow, a mixture between hydrogen and phosphine (PH₃),as a second precursor gas, in the case of GaP growth, or arsine (AsH₃)in the case of GaAs growth, or their mixture, including H₂ as a carriergas and a diluter, in the case of GaAs_(x)P_(1-x) growth, is introducedin the reactor 10 to mix with the GaCl₃ in the second reactor zone 20,called “mixing zone”, with the intention of reacting the mixture on thesurface of the substrate 22 to form, respectively, GaP, GaAs, or aGaAs_(x)P_(1-x) ternary. With regard to FIG. 10B, the process is similarto that presented with regard to FIG. 10A, except that AsH₃ is theternary-forming gas instead of PH₃, and a layer of GaAs is formed on aGaP substrate. GaP substrates and OPGaP templates are available, buthave lower quality and a much higher price than the GaAs substratesshown in FIG. 10A. The lattice mismatch of FIG. 10B is large (the sameas in FIG. 10A but positive) which means that the GaAs layer grows undercompressive stress, which is less favorable than the case of tensilestress. After forming GaCl₃, the most probable chemical reactionsrelated to forming GaP on GaAs or GaAs on GaP are:GaCl₃+PH₃→GaP+3HCl  (eq. 4a)GaCl₃+AsH₃→GaAs+3HCl  (eq. 4b)Other heteroepitaxial cases will have distinct but similar chemistry.

Some typical values for the inner and outer flows of H₂, HCl, PH₃, andAsH₃ related to the HVPE growth of GaP, GaAs, or GaAs_(x)P_(1-x) areprovided in Table 3 below, as an example. However, these numbers arestrictly correlated to growths of GaAs, GaP, or their ternaries and tothe particular configuration of the HVPE reactor presented in FIG. 10A,e.g. a 3-inch diameter reactor tube horizontal quartz reactor. In theseparticular cases the total gas flow was less than 400 sccm (i.e.standard cubic centimeters per minute: the “standard” indicates a gasflow at standard conditions, i.e. at atmospheric pressure and roomtemperature). However, other reactor configurations or processes thatinvolve other materials and precursors may need different flows and flowregimes. In all cases, however, the total gas flow in theclose-to-equilibrium HVPE process will be much less than the huge (manyliters) flows of gases typically used in the far-from-equilibriumprocesses, such as MOCVD or MBE, where the high supersaturation is astrict requirement.

TABLE 3 Some typical values for the inner and outer flows (sccm) of H₂,HCl, PH₃, and AsH₃ during growths of GaP, GaAs, or their ternaries.Inner Flow Outer Flow H₂ HCl H₂ HCl AsH₃ PH₃ 65-75 30-35 100-110 30-3570-80 50-60

With respect to the rest of the growth parameters, all experiments wereconducted with parameters within the following ranges: pressure <10Torr, substrate temperatures 720-740° C. for the growth of GaP(respectively 680-700° C. for the growth of GaAs), and V/III ratios inthe range of 2-3. These ranges provided conditions for aclose-to-equilibrium process at relatively low supersaturation(typically between 0.5-1.0), which is the nature of the HVPE growth.

Growth experiments were conducted homoepitaxially (GaP/GaP andGaAs/GaAs) and heteroepitaxially (GaP/GaAs and GaAs/GaP) on plain“on-axis” (100) GaAs and GaP substrates and on the same (100) substratesbut misoriented with 4° towards (111)B. Growths of the GaAs_(x)P_(1-x)ternary at different x ratios were also performed on both GaP and GaAssubstrates.

An important step in this process, strongly correlated to thisinvention, is related to the way of protecting the substrate 22 (FIGS.10A-10J) from thermal decomposition before growth begins. Suchprotection is generally limited to protecting the substrate from thermaldecomposition as in FIG. 3B, but not necessarily from other, desired,influences of the non-native precursors as presented in FIGS. 3C, 3D,4A-4D, 6A, and 9B-9D). For this purpose when the reactor temperaturereaches about 350° C. the substrate must be kept in either an AsH₃atmosphere, in the case of a GaAs substrate, or in a PH₃ atmosphere, inthe case of a GaP substrate. In the case of heteroepitaxy, we have moreoptions, each of which need to be effective. These include theprotection of a GaAs substrate in a PH₃ atmosphere or, vice versa,protection of a GaP substrate in an AsH₃ atmosphere. In the alternative,we may also protect the wafer by maintaining a mixture of PH₃ and AsH₃gases. In all these cases we should keep in mind that, typically, theprecursor flows, as well as the hydride precursors, e.g. PH₃, AsH₃,H₂Se, H₂Te, SbH₃, etc., are diluted by the carrier gas (H₂); as apractical matter, we use mixtures of AsH₃:H₂, or PH₃:H₂, or AsH₃:PH₃:H₂.One should also bear in mind that the growth of other materials (seeFIGS. 10C-10J) will require distinct chemistries forsubstrate-pretreatment and growth. For example, the growth of ZnSe onGaAs substrates requires pretreatment of the GaAs substrate in anH₂Se:H₂ or an AsH₃:H₂Se:H₂ precursor mixture. To fully explore thecapabilities of the pre-growth stage, we conducted the preheatingprocedures not only in different precursor atmospheres or mixtures ofprecursors, but also using different flow rates, different precursorratios in the mixtures and different regimes of introducing those gasesfollowing the determined schemes. Further experiments of this naturewere continued as the preheating stage was followed by the stage of theepitaxial growth. The later experiments were performed in order toassess the impact of the initial stages of growth on the final layerquality.

Example 2—Growth of ZnSe on GaAs Substrate

This embodiment of the invention is based on hydride vapor phase epitaxy(HVPE) and the heteroepitaxial growth of ZnSe on GaAs substrates, or theopposite case of the heteroepitaxial growth of GaAs on ZnSe substrates.As it was already explained in the text above, many other semiconductormaterials may be grown by this technique and are favored by the proposedapproach. As illustrated in FIG. 10C, the related experimentscorresponding to FIGS. 10A-10J were conducted in a hot wall 3-inchdiameter horizontal quartz reactor 10 positioned in a three-zoneresistive furnace 12. The furnace 12 is not depicted in FIGS. 10B-10J toenhance clarity. A quartz boat 14 positioned in the first zone 16 andfilled with molten zinc, or zinc chloride (ZnCl₂), or a Zn-rich solutionof ZnCl₂, was placed in a one-inch diameter inner tube 18 and a mixtureof hydrogen H₂ and HCl (first precursor) was flowed over the boat 14. Asexplained above, the purpose of the H₂ is not only to carry the HCl butalso to dilute the HCl flow to a desired extent while the HCl flow picksup some zinc (Zn) from the boat 14 to form zinc chloride (ZnCl₂).Another peripheral growth, a mixture between hydrogen and hydrogenselenide (H₂Se) is introduced (as a second precursor) in the reactor 10to mix with the ZnCl₂ in the second reactor zone 20, the “mixing zone”,with the intent to react the mixture on the surface of the substrate 22to form ZnSe. With regard to FIG. 10C, the process is similar to thatpresented with regard to FIG. 10A, except that H₂Se is theternary-forming gas, and a layer of ZnSe is formed on a GaAs substrate.It is noted that ZnSe crystalline substrates large enough (i.e. 1-2 inchin diameter) to create a useful product or OPZnSe templates are notcommercially available; even the best available samples on the market,which are typically no larger than 5 mm×5 mm contain at least severaldomains with different orientations.

ZnSe and GaAs yield a very small lattice mismatch (+0.238%). Thisprovides opportunities to grow crystalline ZnSe on non-native GaAssubstrates. High quality GaAs substrates and orientation-patterned GaAstemplates (OPGaAs) are commercially available at a reasonable price.

Gas flow parameters for the growth of ZnSe on GaAs are similar to thosepresented in example 1. As depicted in FIG. 10C, the formation of ZnSeon GaAs (described below) is likely according to the reaction:Zn+2HCl→ZnCl₂+H₂  (eq. 5a)ZnCl₂+H₂Se→ZnSe+2HCl  (eq. 5b)

The same chemistry may be used for growth of CdSe on InAs because theirlattice mismatch still acceptably small (−0.139%) (see FIG. 13), takinginto account that 2-inch, good quality InAs is commercially available ata reasonable price.

Example 3—Growth of ZnTe on GaSb Substrate

This embodiment (see FIG. 10D) of the invention is based on hydridevapor phase epitaxy (HVPE) and the heteroepitaxial growth of ZnTe onGaSb substrates, or the opposite case of the heteroepitaxial growth ofGaSb on ZnTe substrates. As illustrated in FIG. 10D, the relatedexperiments were conducted in a hot wall 3-inch diameter horizontalquartz reactor 10 positioned in a three-zone resistive furnace 12. Thefurnace 12 is not depicted in FIGS. 10B-10H to enhance clarity. A quartzboat 14 positioned in the first zone 16 and filled with molten zinc (Zn)and zinc chloride (ZnCl₂) is placed in the one-inch diameter inner tube18, and a mixture of hydrogen H₂ as carrier gas and HCl (firstprecursor) is flowed over it. The purpose of the H₂ is not only to carrythe HCl but also to dilute the HCl flow to a desired extent while theHCl flow picks up some zinc (Zn) from the boat 14 to form zinc chloride(ZnCl₂). Another peripheral growth, a mixture between hydrogen andhydrogen telluride (H₂Te) (second precursor) in the case of ZnTe growthis introduced in the reactor 10 to mix with the ZnCl₂ in the secondreactor zone 20, the “mixing zone”, in order to react the mixture on thesurface of the substrate 22 to form ZnTe. With regard to FIG. 10D, theprocess is similar to that presented with regard to FIG. 10A, exceptthat H₂Te is the ternary-forming gas, and a layer of ZnTe is formed on aGaSb substrate. It is noted that ZnTe crystalline substrates largeenough (i.e. 1-2 inch in diameter) to create a useful product or OPZnTetemplates are not commercially available. Even the bestcommercially-available samples available are typically no larger than 5mm×5 mm, and contain at least several domains with differentorientations.

ZnTe and GaAs yield a very small lattice mismatch (+0.083%). Thisprovides opportunities to grow crystalline ZnTe on non-native GaSbsubstrates. However, while 2-inch, good quality GaSb substrates arecommercially available at a reasonable price, orientation-patterned GaSbtemplates (OPGaSb) are not yet available.

Gas flow parameters for the growth of ZnTe are similar to thosepresented in example 1. As depicted in FIG. 10D, the formation of ZnTeon GaSb is likely according to the reaction:ZnCl₂+H₂Te→ZnTe+2HCl  (eq. 6a)if the process is started with ZnCl₂. However, if the process is startedwith Zn and HCl to form ZnCl₂ the most probable chemical reactions willbe:Zn+2HCl→ZnCl₂+H₂→ZnCl₂+H₂Te→ZnTe+2HCl  (eq. 6b)

Regarding the opposite case of the heteroepitaxial growth of GaSb onZnTe substrates (see FIG. 10E), the most probably chemical reactionswill be:2Ga+6HCl→2GaCl₃+3H₂→2GaCl₃+2H₂Te→2GaTe+4HCl+Cl₂  (eq. 7)However, it is less probable for one to choose such growth, because asexplained above good quality ZnTe substrates (or OPZnTe templates) areunavailable in sizes large enough for practical use.

The same chemistry may be used for the growth of GaTe on InAs or CdTe onInSb as far as their lattice mismatches (−0.100% and +0.040%,respectively) are still acceptably small (see FIG. 13), taking intoaccount the availability of the related substrates or templates.

Example 4—Growth of β-GaSe on GaN or AlN Substrate

This embodiment of the invention is based on hydride vapor phase epitaxy(HVPE) and the heteroepitaxial growth of β-GaSe on GaN or AlNsubstrates, or the opposite case of the heteroepitaxial growth of GaN orAlN on β-GaSe substrates. This embodiment is also based on HVPEheteroepitaxial growth of α-GaSe on GaP substrates, or the opposite caseof growth of GaP on α-GaSe substrates, taking into account that goodquality 2-inch GaN and GaP substrates, as well as OPGaN and OPGaPtemplates, are commercially available (with some limited options) whileonly relatively expensive small sized (10 mm×10 mm) GaSe samples areavailable with some limited options.

As illustrated in FIG. 10F, the related experiments were conducted in ahot wall 3-inch diameter horizontal quartz reactor 10 positioned in athree-zone resistive furnace 12. A quartz boat 14 positioned in thefirst zone 16 and filled with molten gallium (Ga) was placed in aone-inch diameter inner tube 18 overflowed with a mixture of hydrogen H₂as carrier gas and HCl (first precursor). The purpose of the H₂ is notonly to carry the HCl but also to dilute the HCl flow to a desiredextent while the HCl flow picks up some gallium (Ga) from the boat 14 toform gallium chloride (GaCl₃). Another peripheral growth, a mixturebetween hydrogen and hydrogen selenide (H₂Se) (second precursor) in thecase of GaSe growth is introduced in the reactor 10 to mix with theGaCl₃ in the second reactor zone 20, the “mixing zone”, in order toreact the mixture on the surface of the substrate 22 to form GaSe. Themost probable chemical reactions related to forming GaSe is:2GaCl₃+2H₂Se→2GaSe+4HCl+Cl₂  (eq. 8a)if the process is started with GaCl₃. However, if the process is startedwith Ga and HCl to form GaCl₃, the most probable chemical reactions willbe:2Ga+6HCl→2GaCl₃+3H₂→2GaCl₃+2H₂Se→2GaSe+4HCl+Cl₂  (eq. 8b)As described above, this variation is related to the growth of thehexagonal β-phase of GaSe on hexagonal substrates (GaN or AlN) or thezinc blended α-phase GaSe (α-GaSe) on GaP substrates. The latter case,however, is much more favorable due to the small lattice mismatch of(α-GaSe) with commercially-available zinc-blended GaP (−0.607%). Forcomparison, the lattice mismatch of β-GaSe with the III-Nitrides (GaNand AlN) is about +17% (see FIG. 10F and FIG. 13). As one can see fromthe above examples, two different phases of the same material (i.e.α-GaSe and β-GaSe) may be grown on two or more different substratematerials (AlN, GaN, GaP). This is another alternative variation of theproposed growth approach. Hexagonal substrates are from materials thatpossess hexagonal symmetry, i.e. having crystal cells that are hexagons.Wurtzite crystals have hexagonal symmetry.

The grown GaSe in this case should be with hexagonal symmetry (forexample β-Ga₂Se₃)—the same as the symmetry of the substrates (GaN orAlN). However, the same chemistry can be used for growth of α-Ga₂Se₃with zinc blended symmetry if the GaN or the AlN substrate is replacedby a substrate with cubic symmetry and small lattice mismatch withα-Ga₂Se₃, for example with a GaP substrate (lattice mismatch −0.607%).The opposite heteroepitaxial cases, for example growth of GaN on thehexagonal β-Ga₂Se₃ (FIG. 10G) or growth of GaP on the zinc blendedα-Ga₂Se₃ is also possible.

With regard to FIG. 10G (the opposite cases of these presented on FIG.10F), for the growth of GaN, the most probable chemical reactions are:2Ga+6HCl→2GaCl₃+3H₂→GaCl₃+NH₃→GaN+3HCl  (eq. 9a)For the growth of AlN, the most probable chemical reactions are:2Al+6HCl→2AlCl₃+3H₂→AlCl₃+NH₃→AlN+3HCl  (eq. 9b)For the growth of GaP, the most probable chemical reactions are:2Ga+6HCl→2GaCl₃+3H₂→GaCl₃+PH₃→GaP+3HCl  (eg. 9c)The chemistry in this case is the same as in the case presented in FIG.10A (see also eq. 3).

In the first and third of these three examples the boat 14 is filledwith gallium (Ga) while in the second example with aluminum (Al). Gasflow parameters for the growth of GaSe, GaSe, and AlN are similar tothose presented in example 1 above. As one can see from the aboveexamples, different materials (e.g. GaN, AlN, GaP) may be grown ondifferent phases of the same material (α-GaSe and β-GaSe). This isanother alternative variation of the proposed growth approach.

Example 5—Growth of GaAs on ZnSe Substrate

This embodiment of the invention is based on hydride vapor phase epitaxy(HVPE) and the heteroepitaxial growth of GaAs on ZnSe substrates. Asillustrated in FIG. 10H, the related experiments were conducted in a hotwall 3-inch diameter horizontal quartz reactor 10 positioned in athree-zone resistive furnace 12. A quartz boat 14 positioned in thefirst zone 16 and filled with molten gallium (Ga) was placed in aone-inch diameter inner tube 18 overflowed with a mixture of hydrogen H₂as carrier gas and HCl (first precursor). The purpose of the H₂ is notonly to carry the HCl but also to dilute the HCl flow to a desiredextent while the HCl flow picks up some gallium (Ga) from the boat 14 toform gallium chloride (GaCl₃). Another peripheral growth, a mixturebetween hydrogen and hydrogen arsenide (AsH₃) (second precursor) in thecase of GaAs growth is introduced in the reactor 10 to mix with theGaCl₃ in the second reactor zone 20, the “mixing zone”, in order toreact the mixture on the surface of the substrate 22 to form GaAs. Themost probable chemical reactions related to forming GaAs is:2Ga+6HCl→2GaCl₃+3H₂→GaCl₃+AsH₃→GaAs+3HCl  (eq. 10)Gas flow parameters for the growth of GaAs are similar to thosepresented in example 1 above. One should bear in mind, however, thatthis case is less favorable than the opposite case of growth ofZnSe/GaAs (shown in FIG. 10C) because, in contrast to GaAs, ZnSe ismostly available in polycrystalline shape. In addition, OPZnSe templatesare not commercially available at this time.

Other heteroepitaxial cases may be determined from FIG. 1. In general,the two materials selected will be as close as possible to each otheralong the horizontal axis (lattice constant), i.e. vertically-aligned.Such examples include, for example, AlAs with GaAs, CdS with InP, andother combinations that may be derived from FIG. 1 or in FIG. 13.

The general rule is to grow these distinct materials at small and, whenpossible, negative lattice mismatches, which is the case when the layergrows under tensile stress. This is considered as the more favorablecondition because tensile strain in the growing layer compensates tosome extent the naturally-compressed substrate surface.

Optimally, the substrate and the growing layer shall have small thermalmismatches, i.e. the two materials expand or shrink in response toheating or cooling at a similar extent with a similar rate. If thethermal mismatch is too great the growing layer, if it is thick enough,may crack. Though not limited to such an arrangement, it is more favoredfor the substrate to be a more mature material, i.e. with more maturetechniques for growth, such as GaAs, GaP, GaSb, InP, InAs, GaN, etc.,which are more readily commercially available at higher quality and at areasonable price. Additional details regarding how to choose suitablycompatible substrate materials and growing layer materials is presentedin more detail further in the text.

Example 6—Growth of AlAs on GaAs Substrate

According to a further variation of the invention, the substrate is GaAs(gallium arsenide), the second precursor is AsH₃ (arsine), and theheteroepitaxial growth is AlAs (aluminum arsenide). This embodiment ofthe invention is based on hydride vapor phase epitaxy (HVPE) and theheteroepitaxial growth of AlAs on GaAs substrates. As illustrated inFIG. 10I, the related experiments were conducted in a hot wall 3-inchdiameter horizontal quartz reactor 10 positioned in a three-zoneresistive furnace 12. A quartz boat 14 positioned in the first zone 16and filled with molten aluminum (Al) was placed in a one-inch diameterinner tube 18 overflowed with a mixture of hydrogen H₂ as carrier gasand HCl (first precursor). The purpose of the H₂ is not only to carrythe HCl but also to dilute the HCl flow to a desired extent while theHCl flow picks up some aluminum (Al) from the boat 14 to form aluminumchloride (AlCl₃). Another peripheral growth, a mixture between hydrogenand hydrogen arsenide (AsH₃) (second precursor) in the case of AlAsgrowth is introduced in the reactor 10 to mix with the AlCl₃ in thesecond reactor zone 20, the “mixing zone”, in order to react the mixtureon the surface of the substrate 22 to form AlAs. The most probablechemical reactions related to forming AlAs is:2Al+6HCl→2AlCl₃+3H₂→AlCl₃+AsH₃→AlAs+3HCl  (eq. 11)Gas flow parameters for the growth of AlAs are similar to thosepresented in example 1 above. There is a very small lattice mismatchbetween AlAs and GaAs: +0.13%. GaAs substrates are available at highquality and reasonable price. High quality orientation-patterned GaAstemplates (OPGaAs) are also available. However, AlAs substrates and OPtemplates are not commercially available, which makes the growth of GaAson an AlAs substrate more difficult.

Example 7—Growth of InAs on GaSb Substrate

According to another variation of the invention, the substrate is GaSb(gallium antimonide), the second precursor is AsH₃ (arsine), and theheteroepitaxial growth is InAs (indium arsenide). This embodiment of theinvention is based on hydride vapor phase epitaxy (HVPE) and theheteroepitaxial growth of InAs on GaSb substrates. As illustrated inFIG. 10J, the related experiments were conducted in a hot wall 3-inchdiameter horizontal quartz reactor 10 positioned in a three-zoneresistive furnace 12. A quartz boat 14 positioned in the first zone 16and filled with molten indium (In) was placed in a one-inch diameterinner tube 18 overflowed with a mixture of hydrogen H₂ as carrier gasand HCl (first precursor). The purpose of the H₂ is not only to carrythe HCl but also to dilute the HCl flow to a desired extent while theHCl flow picks up some indium (In) from the boat 14 to form indiumchloride (InCl₃). Another peripheral growth, a mixture between hydrogenand hydrogen arsenide (AsH₃) (second precursor) in the case of InAsgrowth is introduced in the reactor 10 to mix with the InCl₃ in thesecond reactor zone 20, the “mixing zone”, in order to react the mixtureon the surface of the substrate 22 to form InAs. The most probablechemical reactions related to forming InAs is:2In+6HCl→2InCl₃+3H₂→InCl₃+AsH₃→InAs+3HCl  (eq. 12)Gas flow parameters for the growth of InAs are similar to thosepresented in example 1 above. There is a very small lattice mismatchbetween InAs and GaSb: −0.61%. The availability of both GaSb and InAssubstrates make the presented InAs/GaSb growth and its opposite growthof GaAs/InP possible, although orientation-patterned templates (OPGaSbnor OPInAs) are not currently available.

Without being bound by theory, during heteroepitaxy the relation betweenthe forces that keep the atoms of the substrate in place and the atomsof the growing layer, Ψ_(AA) and Ψ_(BB) from one side and theinterfacial force Ψ_(AB) from the other side is important. Thus, in thecase when Ψ_(AB)>>Ψ_(BB) and Ψ_(AB)≅Ψ_(AA) the interfacial force Ψ_(AB)is strong enough to produce the pseudomorphous growth. As a result,during pseudomorphous growth the lattice of the growing crystal B (e.g.GaP) will be, initially, homogeneously strained to fit to the lattice ofthe substrate crystal A (e.g. GaAs), which occurs at the expense of alinearly-increasing elastic strain. This, depending reciprocally on howlarge the lattice mismatch is, may typically continue to the depositionof no more than about 10-15 monoatomic layers. After this criticalthickness h_(c) (the thickness of the pseudomorphous growth), accordingto the misfit dislocation (MDs) concept, the pseudomorphous growth willbecome energetically unfavorable and the homogeneous strain will bereleased in the formation (in the ideal case) of MDs with a periodicityτ that should depend on the difference between the two lattice constantsa₀-b₀. (Note: Interfacial force is the force across the interfacebetween two phases that keep them together.)

The critical thickness h_(c), in general, increases with the decrease ofthe lattice mismatch, but does not depend linearly on thelinearly-increasing elastic strain. Many other factors, such as the signof the lattice mismatch, the mechanisms of MD formation, etc., must beconsidered to determine this thickness. These factors have beenincorporated into several models related to stress relaxation and misfitdislocation nucleation. Taking into account the sign and the degree ofthe lattice mismatch between GaP and GaAs (−3.57%), and thepreviously-determined periodicity τ of the MDs (about 28 interatomicdistances) it was attempted, theoretically (using FIG. 11), to determinethe expected thickness of the pseudomorphous growth h_(c). FIG. 11illustrates the critical thickness h_(c) as a function of the misfit f.The upper dashed curve with the open squares is obtained by energyminimization with a continuum model (the lower dashed curve) and iscorrelated to a negative (tensile) misfit. The solid curve with thesolid squares is obtained by minimization of the atomistic model forpositive (compressive) misfit. The two solid circles are the MDsimulation results for positive (compressive) misfit of f=+2.5% andf=+5.0%, while the open circle is the simulation result for f=−5.0%negative (tensile) misfit. (Note: that the error bar for the +5.0compressive misfit is smaller than the size of the solid circle symboland that is why does not appear in the figure.) The arrows show the caseof GaP/GaAs at a negative misfit of −3.57%. Thus one can easily see thatat the negative misfit of −3.57%, when the GaP grows tensile-strained onthe GaAs substrate, the critical thickness must be in the range of 5-10monoatomic layers after which the MDs shall start to appear with aperiodicity τ (according to eq. 2) of about 28 monoatomic distances.After such a rough estimation we tried to determine h_(c) and τ usinghigh magnification TEM cross sectional images of areas near theinterface between the GaAs substrate and the HVPE grown GaP layer (seeFIG. 12) by looking for interruptions in the regular lines of atomsparallel and perpendicular to the interface directions.

It was determined that the formation of MD's may be postponed byrelaxing the strain through other strain relief mechanisms, such asroughening of surfaces or formation of other non-uniformities (voids,etc.) near the interface, indicated by the contrast fluctuations (seethe arrows on FIG. 12). These fluctuations may be caused by voids, localstress, or compositional variations. We should also bear in mind thatall theoretical works and simulations (such as those graphicallyexpressed in FIG. 11) assume a flat interface and typically predictsmaller critical thicknesses than those determined in experimentalstudies. There are mechanisms other than the formation of dislocationsfor releasing the strain accumulated as a result of the lattice andthermal mismatches that are thought to be associated with the layerroughness. For example, the formation of voids, above which the averagetensile hydrostatic stress is lower, or the formation of free surfacedue to roughening, may also reduce the strain and thus contribute topostponing the formation of MDs and increasing the critical layerthickness. The lattice mismatch strain may also be accommodated by 90°MDs that are parallel to the interface, rendering them invisible on thecross sectional TEM images. Another possibility is that the interfacialforce Ψ_(AB) is not strong enough to produce pseudomorphous growth, orthat the P atoms replacing some of the As atoms in the GaAs crystal(forming GaAsP) cell, actually, diminish the need for a pseudomorphousgrowth. The above considerations concern HVPE growth techniques in whichthe nature of its fast growth is “seeking” mechanisms for fast relief ofthe fast building strain. This makes postponing the appearance of MDsfor the later stages of the growth not that surprising.

Characterizations

Each pretreated or grown sample was characterized with regard to itssurface morphology and crystalline and optical quality by at leastseveral of the following characterization techniques: cross section andtop layer surface Nomarski optical imaging, x-ray diffraction (XRD),scanning electron microscopy (SEM), atomic force microscopy (AFM),tunnel electron microscopy (TEM), electron dispersion spectroscopy(EDS), optical transmission, and linear and nonlinear opticalabsorption. Each of these characterizations was performed in order toreveal the mechanisms of formation of defects near the interface betweenthe substrate and the growing layer, and also how these defectspropagate in the layer and how they impact the final layer quality,taking into account the impact of the applied pretreatment conditionsand the applied growth parameters during the growth stage.

Characterizations related to the surface morphology, the crystallinelayer quality, and other electrical an optical parameters related tosome specific practical application were used also as feedback to thegrowth process that allowed the determination of the optimal parametersfor pretreating and growth, e.g. substrate and mixing zone temperaturesand the rates of their change, reactor pressure, V-III ratios, gas flowregimes, etc., for a number material cases.

From the dark field TEM images of GaP grown on GaAs sample (see FIG. 2B)it was confirmed that, as it was predicted (see FIG. 2A), the biaxialstrain a during growth of GaP/GaAs can be resolved into a uniaxial shearstress τ on the (111) dislocation glide plane. As a result, edge, screw,and mixed dislocations do appear along the (111) zone; as in the case ofa Burger vector G=002 both edge and mix dislocations are observed, whilein the case of G=002 screw and mix dislocations are observable (see FIG.2B). The good news is that after the first 1-2 μm of growth, which arehighly populated with dislocations, the dislocation density starts tonoticeably decrease with the layer thickness. Thus, as our cross sectionlinear transmission measurements indicated, in the next couple ofhundreds of microns the material demonstrates good IR transparency,which allowed its usage for the intended frequency conversionapplications. Finally, the growing layer is terminated by a smooth topsurface morphology with an average roughness RMS <1 nm for a 1 μm² AFMscanning spot. Such a self-healing effect has been noticed during theHVPE growth of some other semiconductor materials too, for example inthe case of HVPE grown on GaN.

Our measurements indicate that we have determined good controlparameters for engineering of the transition buffer layer, particularlywith regard to its thickness, composition, and quality which allows asmooth transition between two mismatched materials.

We demonstrated that extending the idea of the transition buffer layerto the growth of a thick ternary layer we were able to achieve hundredsof micron thick GaAs_(x)P_(1-x) at different composition andcharacteristics that combine the best nonlinear properties of the twoparenting materials—in this case, the higher nonlinear susceptibility ofGaAs with the lower 2PA of GaP. An example is GaAs_(0.34)P_(0.66). Thusternaries may be a good solution for both device development andimproved heteroepitaxial growth at lower lattice mismatches.

By demonstrating that heteroepitaxy is possible and successful in someless favorable cases (GaP/GaAs and GaAs/GaP) we have opened widely thedoors for other heteroepitaxial cases (ZnSe/GaAs, ZnTe/GaSb, ZnTe/InAs,AlAs/GaAs and even GaP/Si) that provide closer, more favorable latticeand thermal matches. The first in this line, ZnSe was alreadysuccessfully grown by our technique in a large size (quarters of 2-inchwafers) with high crystalline quality (FWHM ˜ 60 arcsec) and 70 μmthickness in a 1-hour growth experiment on a GaAs substrate. Thisprovides an example of the power of the proposed heteroepitaxialapproach because, to date, all commercially-available ZnSe substratesare polycrystalline.

According to FIGS. 1 and 13, there are a number of other favorableheteroepitaxial cases that may be determined by the lattice mismatchbetween the substrate and the growing layer. Such cases include, forexample, CdS/InP (−0.624% lattice mismatch), or vice-versa, i.e. InP/CdS(+0.624% lattice mismatch), AlSb/GaSb (+0.650% lattice mismatch),CdSe/InAs (−0.139% lattice mismatch), GaSb/InAs (+0.620% latticemismatch), AlSb/InAs (+1.273% lattice mismatch), CdTe/InSb (+0.040%lattice mismatch), InSb/CdTe (−0.040% lattice mismatch), HgTe/CdTe(−0.447% lattice mismatch), HgTe/InSb (−0.407% lattice mismatch), oreven CdS/ZnS (−7.064% lattice mismatch). Although the latter mismatch(CdS/ZnS) might be considered as large, there are a number of techniquesand growth modes that can make heteroepitaxy possible even in suchcases. A great example of that is, for instance, the growth of GaN onsapphire, where the lattice mismatch is huge, −33.354%.

As one can see in point of view of lattice mismatch all other suggestedin FIG. 13 heteroepitaxial cases are more favorable than the firstrealized case—GaP/GaAs (−3.574% lattice mismatch), which is a goodprecondition for their success. This was partly proven by successfullyrealizing the next heteroepitaxial case, ZnSe/GaAs (only +0.238% latticemismatch). However, even at larger than those lattice mismatches, thereare a number of techniques and growth modes that can make heteroepitaxypossible even in such less favorable cases.

In one of the example in FIG. 13 (InSe) a substrate material is notspecified. This is because there are some contradictions in theliterature in regards to the InSe crystal structure and latticeconstants. Plus, InSe is a typical 2D material. Such materials can begrown on various substrates as far as they are held to the substrateonly by van der Waals (VDW) forces, a case when the lattice mismatch isnot as important as opposed to the other cases discussed above. Namely,due to this weak interaction the epi-layer, which grows from the verybeginning with its own lattice constant, forms an interface with thesubstrate, the interface has an extremely small number of defects. Thatis why during VDW heteroepitaxy the layer is drastically relaxed whichallows a large variety of different heterostructures, even for highlylattice-mismatched systems. Thus, the proposed approach not only givesnot only the opportunity to heteroepitaxially grow numerous other 2D vander Waals semiconductor materials, such as elemental 2D semiconductors,chalcogenides, phosphides, arsenides, iodides, and oxides, but alsooffers the possibility to combine 3D materials with large latticemismatches by introducing a buffer layer of a 2D crystal for many cases.This is another important alternative variation of this technique.

As taught above, however, the lattice mismatch is not the only importantcriterion when matching two materials in a growth process. First of all,to be practical, the substrate material should be available in arelatively large size (e.g. 2-inch wafers), at a reasonable price andwith high crystalline quality. The availability of such substratesindicates a mature growth and preparation technology. Next to thewell-known, common substrates, e.g. Si, Ge, GaAs, or GaP, some othermaterials, e.g. InAs, InP, InSb, GaSb, and CdTe are also available assubstrates for subsequent epitaxial growth. It makes sense to growmaterials that are either more expensive or not available in large sizewith crystalline quality, e.g. ZnSe or ZnTe, on common or high qualitysubstrates, e.g. growth of crystalline ZnSe/GaAs or ZnTe/GaSb. In thesame way, it is much more reasonable to grow CdSe, which is also notavailable as large crystalline substrates, on InAs, which is availableat high quality and at a fairly reasonable price of about $100 per2-inch wafer. Similarly, it is preferable to grow CdS (about $2,000 per2” wafer) on the cheaper InP (about $400 per 2-inch wafer) than toperform the opposite growth, i.e. InP/CdS. Also to be considered are thegrowths of zinc blende materials (for example GaP) on zinc blendesubstrates (for example GaAs), or wurtzite materials (for example GaN)on wurtzite substrates (for example sapphire) rather than zinc blendematerials with cubic symmetry on wurtzite substrates with hexagonalsymmetry. One also should bear in mind that different crystallographicorientations may provide a closer lattice match to different phases ofone material. Thus hexagonal GaN may be successfully grown on (111) GaAssubstrates, while cubical GaN may be grown on (100) GaAs. The oppositecondition, i.e. that two different phases of the same material may begrow successfully on completely different materials. Thus, as describedabove, α-GaSe can be grown on GaP, while β-GaSe can be grown on GaN (seealso FIG. 13). To simplify the chemistry, it may also be preferable togrow antimonide on antimonide, or selenide on selenide than, forexample, antimonide on sulfide, even if this is at the expense of alarger lattice mismatch. Thus, for example, the case AlSb/GaSb should bepreferred to the case of CdSe/InAs. However, this as many otherrecommendations presented here, shall not be accepted as strict rulesbecause there are cases when growth on the same type of compound mayresult in lower surface quality and a larger number of misfit andtreading dislocations. Some examples: InAs on GaSb have much smallerlattice mismatch (−0.615%) than InAs/GaAs (+7.165% lattice mismatch), orCdSe/InAs (−0.139% lattice mismatch) even though they are semiconductorsfrom different (II-VI and III-V) groups.

One should also bear in mind that, as described above, that from acrystallographic point of view, growth at a negative mismatch thatresults in a tensile strained growing layer is more favorable thangrowth performed at a positive mismatch, i.e. under compressive strain.One explanation of such a preference is that a tensile growing layer canto some extent compensate for the naturally-compressed substratesurface, as well as the fact that tensile growth provides conditionsthat are more favorable for 2D (layer) growth when the growth results inthicker pseudomorphous growth. In contrast, when the growth occurs undercompressive strain, the conditions favor 3D (island) growth, whichresults in rougher surface morphology. That is why, for example, growthof CdS/InP (−0.624% lattice mismatch) should be preferred to growth ofInP/CdS (the same but positive, +0.624%, lattice mismatch.

It should also be remembered that the thermal mismatch between thegrowing layer and the substrate, i.e. the difference between the thermalexpansion coefficients and their thermal conductivities, starts to playa more and more important role with increasing the layer thickness,which can lead to cracking of the growing layer. For example, the 3times smaller thermal conductivity of ZnSe (18 W.m-1K-1) should probablybe taken into account when growing thick ZnSe on GaAs.

Although that the best ternary for the buffer layer should be the onethat is formed by the two parenting materials (for example, GaAsP buffermaterial for GaP/GaAs or GaAs/GaP growths) we should also consider thatone growing material can be fit to the substrate by using a buffer layerformed from a third material having a lattice constant between those ofthe substrate and the growing layer. For example, GaSb was grown by MBEon GaAs by using not only GaSb, but also InAs or AlSb buffer layers.Notice that the growth technique is this example (MBE) is different fromthe method described above (HVPE). Multiple buffer layers and multiplematerials may be used to provide for a gradual transition between twocompletely incompatible materials. A convincing example is the growth ofInSb on a GaAs substrate. The first step is to grow an intermediatelayer of InP (lattice constant 5.8668 Å) on the GaAs substrate (latticeconstant 5.6533 Å). Next is the growth of a second intermediate layer(GaSb-lattice constant 6.0959 Å). Next is to grow a third intermediatelayer (ZnTe-lattice constant 6.1010 Å) on the second GaSb intermediatelayer, and to finish with a thick growth of InSb (lattice constant6.4794 Å).

Of course, the success of such efforts will be greatly improved if theintermediate layers are grown in mixtures of the related precursors (inthe case of the first InP/GaAs transition, an AsH₃+PH₃ mixture in thepresence of Ga and In overflowed by HCl), which will support the growthof ternary or quaternary intermediate layers with a gradually-changingcomposition, ensuring a smooth transition between the two materials.Thus materials that are completely incompatible may be grown on eachother, even with differences in the lattice constants of 10 Å or more.

In summary, the major criteria for choosing a substrate and growingmaterial pair are:

-   -   1. The magnitude and the sigh of the lattice mismatch    -   2. The maturity of the substrate growth and template preparation        technology    -   3. The price and the quality of the available substrates and        growing layer    -   4. The crystallographic structure/symmetry and the chemical        bonds of the substrate and the growing layer    -   5. The difference in the thermal expansion coefficients and the        thermal conductivity of the substrate and the growing layer.    -   6. The toxicity and flammability of the used chemicals.

Precursor Gases and Ternary-Forming Gases

The first precursor gas is usually hydrogen chloride (HCl) diluted tothe desired extent by the carrier gas (usually H₂). The role of theprecursor gas is to pick-up a II or III group element (e.g. Ga, Al, Sb,Cd) from an open boat or a bubbler, and with it to form a metal-chlorinecompound, called “precursor” which is delivered to the mixing zone,making it available to participate in the growing process.

The ternary-forming gas, i.e. second precursor gasses, is usually ahydride of a V or VI group element (AsH₃, PH₃, H₂S, SbH₃, etc.) dilutedto the desired extent by the carrier gas (usually H₂). The role of theternary-forming gas, which is actually the precursor of the V orVI-group element, is to be delivered to the mixing zone, making itavailable to participate in the growing process. We call this precursor“ternary-forming” because the reactions between the precursor gas andthe ternary-forming gas on the foreign substrate may result in theformation of ternary islands which eventually coalescence, forming acontinuous ternary buffer layer.

Alternative names: The precursor gas may be called “precursor of the IIor III-group element” or “first precursor”. The ternary-forming gas maybe called “precursor of the V or VI-group element” or “secondprecursor”.

We reserve the right to use the same names for mixtures from the relatedprecursors.

By demonstrating that heteroepitaxy is possible and successful at largerlattice mismatches without using a patterned template in one step growthprocess (preceded by substrate pre-growth pretreatment) we haveeliminated the need for growth on patterned substrates at largermismatches, or the preliminary deposition of a thin MOCVD or MBE layer,or even the HVPE deposition of a LT lower quality buffer layer. Thedisclosed parameters of heteroepitaxy, e.g. the thickness of thepseudomorphous growth and periodicity of the misfit dislocations, forsome particular cases have established clear criteria by whichadditional cases of heteroepitaxy may be deemed favorable.

The invention described herein is an innovative approach for pre-growthin-situ treatment of a substrate and the subsequent optimized thick HVPEheteroepitaxial growth on the substrate as a continuation of thepretreatment. The inventive approach applies discovered processparameters that secure a smooth transition between two differentmaterials and the process is flexible enough to adapt these parameterseven at relatively large lattice and thermal mismatches. The evidenceprovided herein regarding the successful heteroepitaxial growth ofmaterials which are disfavored according to the known prior art supportsthe application of the inventive process over a wide range ofsemiconductor pairs having differing degrees of lattice or thermalmismatch.

As mentioned above, the invention consists broadly of two steps: (1)pre-growth treatment of the substrate in order to initiate thereplacement of V-group atoms of the substrate with V-group atoms of thelayer intended to be grown, and (2) heteroepitaxial growth on thepretreated substrate. The proposed approach allow plenty of opportunityfor applying different process parameters, using different temperatures,start times, durations, flow rates, etc. The invention allows one tofreely adjust the process parameters and to accommodate materials toeach other.

The expectations are that the proposed approach will have an instantimpact on several different R & D areas such as development of frequencyconversion laser sources, the solar cell industry, and inoptoelectronics. Each of these areas will facilitate the developments ofmore specific subareas. For example, the development of new frequencyconversion laser sources may result in a great number of military (IRcountermeasures, laser radar, IR communications) and civilianapplications in areas such as security (new type airport scanners,remote sensing of explosives or biological agents), environmentprotection (UV water purification), energy conservation (LED), highenergy physics (high power lasers with use, for example, in nuclearsynthesis), industry, medicine, and science. Similarly, developments ofoptoelectronics will have a strong impact on development ofall-optically-communicating devices, as well developments in the solarcell industry based on heteroepitaxy can give us a better chance fordevelopment of multilayer, multiphoton, portable solar cells, or newmulticolor detectors that cover large spectral ranges, including bothatmospheric windows of transparency, etc.

Alternative Variations:

1. Growths on different crystallographic orientations may provide closerlattice match to different phases of one and the same material.

Examples: Hexagonal GaN can be successfully grown on (111) GaAssubstrates, while cubic GaN can be grown on (100) GaAs.

2. Two different phases of the same material can grow successfully oncompletely different materials.

Examples: α-GaSe can be grown on GaP substrates while β-GaSe may begrown on GaN substrates (see also FIG. 13).

3. A buffer layer comprising a third material that has a latticeconstant between those of the substrate and the growing layer.

Examples: GaSb can be grown on GaAs by using not only GaSb, but alsoInAs or AlSb buffer layers.

4. Multiple buffer layers from multiple distinct materials for a gentleand gradual transition between two completely incompatible materials.

Examples: Grow first an intermediate InP layer (lattice constant 5.8668Å) on a GaAs substrate (lattice constant 5.6533 Å); continue with thegrowth of a second (GaSb) intermediate layer (lattice constant 6.0959Å); grow a third ZnTe intermediate layer (lattice constant 6.1010 Å) onthe second GaAs intermediate layer, and finish with a thick growth ofInSb (lattice constant 6.4794 Å).5. Heteroepitaxial growth of 2D van der Waals semiconductor materials.Examples: elemental 2D semiconductors, InSe and other selenides,phosphides, arsenides, iodides, oxides, and chalcogenides.6. The proposed approach is applicable not only for III-V compounds(e.g. GaAs, InAs, AlAs, GaSb, InSb, AlSb, GaP, InP, GaN, AlN, etc.) butalso for II-VI (e.g. ZnSe, CdSe, CdTe, HgTe, ZnTe, CdS, etc.) and evenIII-VI (GaSe, InSe, GaTe) or II-V (ZnSe) semiconductor compounds. At thesame time it is not a strict rule to grow one group's materials onsubstrates from the same group (e.g. a III-V group material on a III-Vgroup substrate, etc.) because mixed cases may be more favorable—for,instance, the case of the already realized case of ZnSe (II-VI group)grown heteroepitaxially on the III-V group GaAs substrate.7. The combination of one close-to-equilibrium growth technique (HVPE)with one far-from-equilibrium growth technique (MBE, MOCVD), makes itpossible to grow heterostructures which appear impossible according tothe prior art, including growths on common substrates, such as Si or Ge.Examples: 1) Growth can start with thin MOCVD or MBE growth of GaP on Siand continue with thick HVPE growth of GaP on the GaP intermediatelayer, which may be continued with thick HVPE growth of GaAs on the HVPEGaP layer, and then continued further with thick HVPE growth of ZnSe onthe HVPE grown GaAs, etc.;2) Growth can start with thin MOCVD or MBE growth of GaAs on Ge andcontinued with thick HVPE growth of GaAs on the GaAs intermediate layer,which may be continued with thick HVPE growth of GaP on the HVPE GaAslayer, and then continued further with thick HVPE growth of α-GaSe onthe HVPE grown GaP, etc.

While the present invention has been illustrated by a description of oneor more embodiments thereof and while these embodiments have beendescribed in considerable detail, they are not intended to restrict orin any way limit the scope of the appended claims to such detail.Additional advantages and modifications will readily appear to thoseskilled in the art. The invention in its broader aspects is thereforenot limited to the specific details, representative apparatus andmethod, and illustrative examples shown and described. Accordingly,departures may be made from such details without departing from thescope of the general inventive concept.

What is claimed is:
 1. A method of performing heteroepitaxy, comprising:exposing a substrate to a carrier gas, a first precursor gas, a GroupII/III element, and a second precursor gas, to form a heteroepitaxialgrowth of β-GaSe (gallium selenide) on the substrate; wherein thesubstrate comprises GaN; wherein the carrier gas is H₂, wherein thefirst precursor is HCl, the Group II/III element comprises Ga; andwherein the second precursor is H₂Se (hydrogen selenide).
 2. A method ofperforming heteroepitaxy, comprising: exposing a substrate to a carriergas, a first precursor gas, a Group II/III element, and a secondprecursor gas, to form a heteroepitaxial growth of β-GaSe (galliumselenide) on the substrate; wherein the substrate comprises AlN; whereinthe carrier gas is H₂, wherein the first precursor is HCl, the GroupII/III element comprises Ga; and wherein the second precursor is H₂Se(hydrogen selenide).